"Souls are one of the things that make each character unique in RIFT. Each class has the choice of eight different Souls to choose from. Players can have up to three Souls active at a time. Souls determine the skills and abilities of the character. The first quest you will receive at level 1 will be your first Soul choice. These Souls can be switched out as you acquire new Souls."
Source: Rift Support Center: Souls
The above quote sums up the Soul tree (talent trees for other MMO gamers) pretty well. But exactly how do you use them?