Feral (also known colloquially as cat or kitty) is the melee DPS specialization for the druid class. It's styled similarly to a rogue, but instead of sneaking around and using poisons, you just make things bleed. Copiously.
A feral druid's primary resource is energy. It is a 100-point pool that regenerates at 10 energy/sec, in and out of combat, though this rate can be increased with haste, and is used to power your basic attacks. This energy is used to power a variety of attacks, many of which build combo points (abbreviated cp's). Combo points are stored on a target and can stack up to 5; if you change targets and use another cp-generating attack, the stored points are lost. These points are used for powerful finishing moves.