What is the difference between EPM and APM?

The notes for patch 1.4.3 state:

• APM values displayed on the Leader Board are now calculated by counting every click and command that has been issued by the player.
• EPM values displayed on the Leader Board are calculated by counting only actual commands issued by the player (Effective Actions Per Minute).

EPM is always smaller than APM since effective actions are a subset of all actions.
Time is measured in game time (aka Blizzard time), and it is not measured in real time.

As mentioned at a Team Liquid forum post, camera location commands (aka screen bookmarks) are not included in the calculations of APM and EPM.

Note that third party analysis tools often measure APM/EPM slightly differently to the Starcraft 2 software.

Currently (as of version 1.4.3.21029) there is a bug where EPM and APM values are reversed when viewing a replay. We should expect a fix sometime in a future patch.

EPM = effective actions per minute is every valid action you make. So moving your marine will count but pressing a control group won't (Starcraft 2 default).

APM = actions per minute counts every click you make, like pressing a control group or moving a marine (old Brood War style). It should always be greater than or equal to EPM.

Note that (as of April 2012) these metrics appear reversed in replays.

APM = Actions Per Minute

EAPM or EPM = Effective Actions Per Minute

For the APM calculation all your actions are accumulated, every click and every key press is counted as 1 action, and divided by the game duration. There are a few excretions to this rule, Shift and Ctrl are not counted as an action, but Shift + 1 and Ctrl + 1 are.

This means that APM measure your average control speed, in other words how fast you can click your buttons.

Blizzard introduced the EAPM to filter out some actions they think are unimportant. While the calculation is basically the same there is a filter that removes "redundant" actions before the accumulated value is divided by the game duration.

Here are some examples what Blizzard thinks is redundant:

• Issuing the same command more than once in a row. (e.g. Move, Move, Move = 1 command; Move, Stop, Move = 3 commands)
• Changing group selection too fast. There is a treshold of about 0,5 seconds and if you switch your groups faster than that it is considered spamming. (e.g. switching between control groups 1 and 2 faster than 0,5 s = 1 click; switching between control groups 1 and 2 slower than 0,5 s = multiple clicks)
• ...

The exact Blizzard calculation is unknown. But SC2Gears does almost the same thing and it is well documented here.