Combo point

Specific to the druid (in Cat Form) and the rogue class, Combo Points are accumulated as special attacks are performed against an enemy, then spent on other special attacks. The number of combo points obtained is represented by small red circles on the right side of the enemy portrait in the UI. Attacks which generate combo points are sometimes referred to as builders, and attacks which consume combo points are referred to as finishing moves or finishers. The more combo points the player has built up against an enemy, the greater the effect of the finishing move.

Mechanics
Each finishing move requires at least one Combo Point. The effects of finishing moves are more dramatic based on the number of Combo Points accumulated. The Combo Points built up in battle against one target cannot be transferred or used against to a different enemy. As of Patch 2.0.3, the rogue will not lose Combo Points upon switching targets until a point has been added to another target. For the druid, switching forms will not reset your Combo Points as long as you keep the same target. Once the finishing move has been successfully executed, the Combo Points reset to zero.

Additional information
Rogue talents and Druid Feral Combat Talents can improve combo point generation. Many players choose to install addons to help manage their combo points.