Orchid have two distinct patterns of growth : monopodial & sympodial.
Monopodial orchids, such as Phalaenopsis and Vandas, have a single growth, typically an elongated stem which may be fairly short (i.e. Phalaenopsis) or reach several feet long like some Vandas, Renantheras or like the liana like Vanilla.
Sympodial orchids, such as Cattleyas, Oncidiums, Cymbidiums, Paphiopedilums,  have several, sometimes dozens of successive growths.
In sympodial orchids the life of each growth is determinate (meaning it will last 3, 4 or 7 seven years) but the life of the plants may be, in theory, unlimited.  The Manual of Cultivated Orchid Species (by Helmut Bechtel, Phillip Cribb and Edmund Launert, MIT Press) says : “the longevity of orchid plants in cultivation is still something of a mystery, for several plants in the orchid greenhouses at Kew are over 100 years old and are still thriving and flowering regularly”.          
The roots of orchids perform two basic functions, and in some orchids, they perform a third one. First they secure the plants where they grow.  Roots of epiphytic orchids  once attached on the bark of the tree they grow on, or to the clay pot they grow in, are nearly impossible to detach.
The second function is to provide the plant with water and dissolved mineral salts.
Roots of terrestrial orchids are relatively simple.  They originate at the base of the stem, they are usually thin, long, fibrous and rarely branched.  Sometimes, as in the case of Paphiopedilums, they are densely haired (or tomentose) so that they can absorb moisture from the tinniest particles of growing medium.
Roots of epiphytic orchids tend to be more complex, and for good reason, as water supply can be erratic, and this water supply will often contain very limited amounts of mineral salts.
A distinctive feature of the roots of epiphytic orchids is their silvery to gray color.  This is due to the velamen which consists of a single or of several layers of epidermal cells.  The velamen covers all the root system except for the short terminal tip of the roots. The role of the velamen is to absorb moisture from the ambient atmosphere, and, may be, to protect from cold or heat.
A third quite remarkable function of the roots of epiphytic orchids is their capacity to photosynthesize.  In some species the roots have altogether taken this function from the leaves. Genera such as Campylocentrum and Microcoelia are completely leafless, and in an extreme case, such as the genus Taenuiphylum, roots are flat, green and look very much like leaves.
Roots of epiphytic orchids are almost constantly exposed to the air.   And though the ambient humidity may be high and, in some cases, tropical rains may be a daily event, these roots are never immersed in water, certainly not for any extensive period of time.
Roots of epiphytic or lithophytic orchids need a lot of air to function. Its is extremely important to know this as most orchids are killed because of excessive water (which chases the air).  
The most common containers / supports for growing orchids are:

Plastic pots
Most commercial growers use plastic pots because they are inexpensive, they are lighter and they are easier to store; because they hold water for a longer period than other pots / containers, because mineral salts (from water and fertilizer) will not adhere to them, and because roots will not get attached to them.
Plastic pots are excellent containers for growing orchids.  Their only draw back, is that some plants, notably Dendrobiums, might get top heavy in them.
Green plastic pots are the ones most commonly used plastic pots. Lately clear plastic pots have become more widely available. Advocates of clear plastic pots claim the light transmission of clear plastic pots enable roots to photosynthesize.
If you are going to use plastic pots, look for pots with a fair number of drainage holes ( 4 to 8 holes on  3” to 4” pots,  8 to 12 holes on 5” to 6” pots,...). .
Clay pots
The advantage of clay pots is the stability due to their weight and their porosity which allows the potting material to dry faster. Of course, drying faster can also be a disadvantage. A possible disadvantage is that roots tend to attach themselves to the pot.
Clay orchid pots
Have either holes or slits on their sides to allow more air circulation than regular clay pots.  As a result, they dry even faster than regular clay pots.
Vanda baskets
Are used mostly for Vandas and vandaceous orchids, but can be used for most orchid genera.
Most Vanda baskets are made of cedar or teak wood. Unfortunately the cedar baskets available today tend to decay in a couple of years.  Teak baskets are expensive and because teak trees are being depleted it is not ecologically friendly to buy teak baskets. Fortunately plastic Vanda baskets have made their appearance.  The ones I saw (4” and 8”) are made of sturdy plastic that should last forever if we recycle them.  
Cork slabs
Are used for mounting orchids.  Pieces of cork range can be as small as 2” by 3” or as large as 12” by 24”.
Some orchids will only thrive when mounted on a piece of cork or on a tree fern slab or a piece of driftwood, but many that grow fine in pots will also thrive on a piece of cork and it makes for a much more natural and interesting look.  
Tree fern plaques
Are flat, and come in different sizes (4” by 4”,  4” by 6”,  8” by 8”,...).  Like cork, they are also used to mount orchids.  Although easier to cut than cork slabs I am not crazy about tree fern slabs because some of them are so dense they barely absorb any water while other appear to fall apart as you handle them.  
An alternative for cork that also can make for some very intriguing and interesting “compositions”.  
Understanding the properties of potting materials will help us select the potting material that best meets the other criterias.
What potting materials for orchids must do is:
1 - hold the plant in place,
2 - hold enough moisture (water) for the needs of this particular orchid,
3- provide an environment that will enhance the development of roots (aeration).
Most potting materials for orchids do not provide any nutrients.  We add the nutrients in the water via the fertilizers we use and that’s how they are made available to the plants.
There are many potting material for orchids, such as:
• orchid bark,      
• sphagnum moss,
• tree fern,
• osmunda fiber,
• coconut chunks,
• coconut fiber,
• lava rock,
• charcoal,
• pieces of cork,
• peat moss,
• rockwool, ...
There are many other potting materials. A grower I know in Hawaii uses the shells of macadamia nuts (which he gets for free), another grower in British Columbia uses straight perlite. I heard of a French grower who uses straight Styrofoam peanuts and many growers in Thailand use 1/2 shells of immature coconuts,..
Today in the United States the most commonly used potting materials are orchid fir bark and sphagnum moss.
Orchid bark
Orchid bark is an excellent material. It is easy to use, it will not hold excessive water and, under normal use will not need to be refreshed for about 2 years.
Orchid bark (which usually is the bark from redwood or Douglas fir), comes in 3 sizes (sizes are also referred to as ‘grades”): small size (also known as “seedling” size), medium and large (or coarse) size.  
The sizes used are mostly the seedling size and the medium size.
Bark is rarely used alone.  Most growers add to it one or more of the following:
- perlite,
- sponge rock (which is expanded perlite),
- charcoal (horticultural grade),  
- sphagnum moss,
- tree fern,
- peat moss,...
Perlite and sponge rock are used to create more air space in the mix .
Charcoal is used to absorb harmful materials that may be in the water.  
Sphagnum or peat moss are used to increase the water holding capacity of the mix.
Sphagnum moss
Sphagnum moss (premium grade which is long fibered and completely free of debris and other materials)  is an excellent material which is widely believed to have fungicidal properties. It is more expensive than orchid bark.    
Sphagnum moss holds a lot more water than bark, is not as easy to use and needs to be refreshed every year.
Sphagnum moss comes mostly in two grades: fine (or seedling grade) and long fibered.  
Tree fern
Tree fern, from the roots of a fern called “tree fern”, is an excellent material that is relatively easy to use and will easily stay fresh for 3 years. The material is relatively expensive.
There are 2 varieties of tree fern: one is sort of light brown and flexible, is mostly available in Hawaii and is called Hawaiian tree fern, also known as “hapu”, the other is dark brown and rigid and comes mostly from Central America. The latter is what is typically referred to as “tree fern” and, unless otherwise noted, when we mention “tree fern” we mean the rigid one from Central America.
Medium tree fern holds just about as much water as medium fir bark (fine tree fern holds more than fine fir bark), but by far not as much as sphagnum moss. It is more expensive than orchid bark.
Tree fern also comes in 3 sizes: fine (or seedling grade), medium and coarse.
Osmunda fiber
Was a choice material in decades past.  Nowadays it is not as readily available and it is expensive. Furthermore, it comes in relatively large chunks that you have to cut into about 1/2” chunks in order to use it.  Great exercise for the wrist but who has the time ? Furthermore, when using osmunda fiber you must make sure the fibers are aligned vertically so as to allow the water to drain.
Coconut chunks
Coconut has been widely used in Asia where it is readily available and is a renewable resource.
In the last few years it started being used in the US Coconut comes either in chunks (small and medium size) or in long fibers.
Coconut is more expensive than bark but it will last up to 5 years and is relatively easy to use.  
One drawback of the coconut chunks is that, based on literature we read, it can be very high in sodium when first used (some say “depending on the source”).  As it ages it apparently looses much of its sodium content.
Our experience using coconut chunks as a potting material is limited, but from this limited experience we found it holds the water much longer than bark and therefore we would suggest caution when using it.  
Coconut fiber
Coconut fiber, as far as we know, is not widely used to grow orchids.  We use a thin layer of it to line Vanda and wire baskets to prevent other potting materials from falling out of the basket. 
We also use it to “stuff” Vanda baskets when potting vandaceous orchids (Vandas and related) so as to hold just a little bit of moisture and to help hold the plant in place.  Use it sparingly, and “fluff” it because if it is too dense it will stay wet and Vandaceous orchids hate this.
Lava rock
As far as we know it is mostly used by growers in Hawaii where it is plentiful.
We do not use straight lava rock as a potting material. We use a limited amount of lava rock as part of our semi-terrestrial mix (see potting mixes).
In the US charcoal is rarely used as the main or sole ingredient of the potting material. Some growers add charcoal to their potting material because charcoal absorbs toxins that may be present in the water and, as Eric A. Christenson wrote in his book “Phalaenopsis - a monography” charcoal will also absorb toxins released by the roots of plants.
Charcoal does not degrade easily so it will retain it’s ability to aerate the potting material. If you are going to use charcoal, make sure to only use horticultural grade charcoal.  
Pieces or cork / cork from bottles
Cork comes in relatively large pieces (slabs) and to use it as a potting material you’ll have to cut it to useable pieces, may be about 1/2” in size, which is difficult, and could be dangerous. Same will go for cork from (wine) bottles.
We only use relatively large pieces of cork slabs (2” by 3”, 4” by 6”, 6” by 8”...) to mount orchids.
Peat moss
We have no experience at all using this neither as the main ingredient nor as an additive. From what we read about peat moss it has a high water retention capacity and it does stay relatively intact for several years. Some growers include it as part of their mix.
Rockwoll is an inert material that looks like dirty cotton.  There are 2 varieties of rockwool: one absorbs water, the other repels water.
Ten years ago or so we used water absorbent rockwool as part of our potting mix but gave up on it because it holds too much water and was difficult to mix. But the worse was its propensity to collapse which  reduced the air space in the potting material.
Aliven (man-made clay pellets)
Several years ago we experienced with Aliven because it is inert, practically indestructible and it is easy of use (we could say the same about lava rock).
At first we thought we had a winner, but as plants stayed longer in it we realized new roots had a pronounced tendency to grow out of the pot and not in it. After a year or so we gave up on it.


-plastic pots
-clay pots
-orchid clay pots

-Vanda baskets
-wire baskets

-cork slabs,
-tree fern plaques,
-pieces of driftwood.