These products contain a root fungi called mycorrhizae, which is packaged in suspension in some kind of medium that will store for up to two years. It is used to help both broad-leaf trees and evergreens. These products are used when planting new plants, transplanting plants to a new place, or on plants already in place. The idea is to get the mycorrhizae down into the root zone of a plant so they can attach themselves to the root system and start doing their good works.
When the mycorrhizal fungi colonize the plant's root system, they create a network that increases the plant's capacity to absorb more water and nutrients such as phosphorus, copper and zinc. This process in turn enhances growth and favors rapid development of roots and plants.
Different groups of plants use different kinds of mycorrhizae, sort of keeping things in each family. On the left are descriptions of each product for each group of plants.
For more information about mycorrhizae go to Yardener's Helper and check out the file Adding Root Fungi To The Soil