A class trainer is an NPC who is able to train your character its many class-related skills (abilities, powers, and spells). They are able to teach new skills every 2 levels (even levels) until level 60, and every level for levels 60-80. For the most part, class trainers only exist in non-Northrend parts of Azeroth.
A class trainer can also untrain your talents (making you untalented) for a price. Untraining costs 1 the first time, then increases by 5 thereafter for a maximum cost of 50. The costs are reduced by 5 each month to a minimum of 15.
When using the tracking system with the minimap they will appear with a blue book () at their locations.
The trainer page has links to a comprehensive list of trainers for each class.
In Mists of PandariaThis section concerns content exclusive to Mists of Pandaria.
As of Patch 5.0, players no longer need to visit class trainers to learn new abilities as they are automatically learned when the required level is reached.