Sphagum Moss is a plant that grows on the surface of a wet soil or swamp. This plant usually thrives on wet climates such as New Zealand. Sphagum Moss is a fine substrate and it can hold water very well which makes it an excellent potting material for young orchids. Some species of Sphagum Moss can hold up to 20 times it's dry weight in water making it very popular as a soil conditioner.
Pros: Retains water and air; readily available
Cons: Can retain too much water if packed tightly in the pot or after it starts to decompose
Coconut Coir is a coconut fibre extracted from the husk of coconut. Coir is the fibrous material found between the coconut's coat and inner shell. This retains water very well that's why it is commonly used as potting medium. You can get coir either as bricks composed of little chunks, which comes apart when the brick is soaked and can be used by themselves or mixed with other potting mix. It can also be purchased as bricks that turns into fiber when wetted, somewhat like peat moss.
Pros: Retains water well; decomposes slowly
Cons: Does not drain as very well
The most common Orchid Bark is Fir Bark. This usually comes in three grades (fine, medium, coarse). The finer the bark, the slower it dries but also lesser airflow. So for orchids that don't like to dry out, such as Paphiopedilum, use a fine grade. For orchids that like a lot of airflow and want to dry quickly, such as Cattleyas, use a coarse mix.
Pros: Easy to obtain; inexpensive; available in many grades
Cons: Can be difficult to wet; decomposes relatively quickly
Lava Rock - Lava rock is an excellent base for an inorganic rock mix. Many times if you get a Dendrobium you will notice that it has been potted in either black or red volcanic rock. Lava rock has good water retention with excellent aeration. Because it is inorganic, the mix will not break down and orchids that resent repotting can be left in their mix longer.
Pros: Never decomposes; drains well; excellent aeration