Actions per Minute
From Liquipedia Starcraft Wiki
Actions per Minute (APM) is a measurement for the speed at which a user is able to interact with the game. Typical APM ranges from 20 for absolute beginners to about 300 for progamers. As StarCraft is almost always played on fastest speed, APM from replays is usually calculated using that speed. Each command such as select, move, attack which is recorded in a replay counts as an action. Thus an action can consist of several clicks/keypresses.
 APM and Skill
It is controversial how APM and skill relate. Generally high skill requires high APM, but having high APM does not mean you are skilled. It should be noted that the very best pros, such as Jaedong, Bisu, and Flash often have extremely high APM (300 or above), but the fastest Progamers ever are not in the top tier.
 How to Improve APM
APM is not easily improvable and is another aspect of StarCraft that takes time to master. Just as people slowly learn strategies and obtain game sense, the same applies for APM. However, it should be noted that APM is not vital to the beginner as the quality of the actions is far more important than the quantity of the actions. Generally, the most effective way to train APM is to practice often so that you can improve your skill in all aspects of the game.
 Variants of APM
Every Action counts.
Shown by BWChart. Removes some actions which BWChart considers invalid. Usually very close to the normal APM and thus rather useless.
Effective APM. First used by bwRepInfo it removes actions which are probably spam. This includes selects without subsequent commands, assigned but not used hotkeys, move spam to a fixed position, etc. What actions are removed differs from program to program, but usually when talking about EAPM the one displayed by bwRepInfo is meant.
 Macro APM
Actions related to macro, such as creating buildings and units, managing the base, and all selects which are followed by one of these commands.
 Micro APM
Actions related to micro, such as attacking and moving your units and all selects/hotkeys which are followed by one of these commands
 Measuring APM
There are basically two ways to measure APM. One is analyzing a replay after the game has ended. The other is used while the game is in progress and might even alert you if your APM falls below a certain threshold.
 From a Replay
The classic program for analyzing replays. Calculates total APM, MacroAPM, MicroAPM. Can plot the APM against the gametime with a choosable dynamic.
It displays the Effective APM(EAPM)of all Players.
Shows your APM ingame. For observers and in replays it displays the APM of all players. Alerts you if your APM falls below a configurable threshold. The APM for a certain time is calculated using an exponential decay of older actions with a fixed decay parameter of about one minute.