From baseball. A run that would have scored, regardless of what defensive errors may have occurred to prolong the inning. Those runs that score after what would have been the third out (for three outs are all you get in an inning) are called - you guessed it! Give that man, woman, child, action figure, or small woodland creature a cigar! - an unearned run.

I've probably done a poor job of explaining it (actually, I've gotten it wrong, haven't I?), so I'll grab the beginning of Rule 10.18:

"An earned run is a run for which the pitcher is held accountable. In determining earned runs, the inning should be reconstructed without the errors (which include catcher's interference) and passed balls, and the benefit of the doubt should always be given to the pitcher in determining which bases would have been reached by errorless play. For the purpose of determining earned runs, an intentional base on balls, regardless of the circumstances, shall be construed in exactly the same manner as any other base on balls..."

That's better. Right?