The most commonly and popularly grown orchids (family Orchidaceae) are actually epiphytes, which means they grow in trees and not in the ground. This means that they need a pourous potting mixture, if grown in pots. Commonly used in creating an ‘orchid mix’ is a combination of pine bark, perlite (an amorphous volcanic glass), and horticultural grade charcoal (to absorb toxins), among many other additives orchid enthusiasts use.
Epiphytic orchids have specialized roots where the older parts, which turn a silvery white, are covered with a modified spongy epidermis called velamen, which serves to absorb humidity and water from the air and substrate. Orchid roots need air circulation. This is why if you plant an epiphytic orchid in soil it will die. So don’t!