very class has three talent trees that are organized by the focus of the talents in that tree. Mages, for instance, rely primarily on their
spells, which are broken up into the arcane, frost, and fire schools - which are also the names and focuses of their three talent trees. The talents in a particular tree will tend to improve the abilities associated with that tree - for example, Improved Frostbolt in the Mages' Frost tree. You can switch between trees by using the tabs at the bottom of the
Talents window.
Talent trees are structured in vertical tiers. A minimum number of talent points must be spent in a tree to learn talents below the first (top) tier. Sometimes, specific lower tier talents must be learned as a prerequisite to learning higher-tier ones. A talent that is grayed out cannot presently be learned, either because you have no talent points to spend or because you do not meet the prerequisites to take that talent.
Starting at the "11 point" talent (thusly named because it first becomes available with the minimum 10 points spent in that tree), and every 10 minimum points thereafter invested in that tree, classes can learn a special talent that typically grants an entirely new class ability. Examples of this include Circle of Healing, the Priest's 41 point Holy talent, Tree of Life, the Druid's 41 point Restoration talent, and Adrenaline Rush, the Rogue's 31 point Combat talent. Typically, these abilities must be activated to use, and boost the power of your character.