The Tome of Channeling introduces three new descriptors; the animal, plant, and weather descriptors. These descriptors are applied to incantations of the nature sphere, and are used to classify these incantations more easily.
While other books do not use the aforementioned descriptors, you can apply them to existing spells, powers, and other effects. For example, the control weather spell would be an ideal candidate for the weather descriptor. The Appendix shows you which spells and powers in the Player’s Handbook and Expanded Psionics Handbook are eligible for the animal, plant, and weather descriptors.
The use of these descriptors also make it easier to create feats and other custom content that are based around effects with these descriptors. For example, you could create a prestige class that is based on spells with the plant descriptor, without having to explicitly name every spell that is related to that prestige class.