Actions per minute (APM) is a measurement of a player's load-handling capacity and is one of the metrics for judging player skill.
Ideally, APM includes only "productive" actions. APM may be artificially inflated by superfluous repetitive actions.
In StarCraft, there is very little idle-time during a game where a player is inactive, so it is often advantageous to be able to perform actions faster than your opponent(s). Some strategies even focus on increasing the amount of things your opponent must do (usually by increasing the things he needs to respond to by attacking in multiple spots simultaneously).
Actions include selecting a unit/building, ordering it (attack, move, patrol, gather, repair, morph, build, warp, stop, hold position, cast spells, burrow, etc.), training a unit, etc. For example, selecting a hatchery, selecting the larvae (zerg hatcheries, lairs and hives have a command to select all larvae of the building), and training 3 hydralisks constitutes 3 actions.
In StarCraft II, APM can be measured in observer mode. As of patch 1.4.3, a related measurement, "Effective Actions per Minute" (EPM), was created. It counts actual commands and not just "spam clicks".
|>200||Proficient player with superfluous actions|
BWChart, the most popular tool for measuring APM.