When is this allowed?

Today I encoutered this, and I'm wondering why, and when this is allowed. 

En passant; you should google it, and don't feel embarassed.  Everyone has to learn...

Here's a link:

http://www.chess.com/chessopedia/view/en-passant

That's en passant (French for slidey pawn)--an old French trick move that somehow got into the real rules.

i like me some En Passant :)

It's en passant. http://en.wikipedia.org/wiki/En_passant "In passing." When a pawn makes a double move, on the next move and (only on the next move) another pawn (and only another pawn) can capture as if it made only the single move. It captures the pawn on its way through the double move "in passing." It is allowed because otherwise pawns would be able to get by each other without being subject to capture by timing the double move just right which would change the game pretty significally. I think it was added around the same time as the pawns double move in order to fix that problem the double move rule created, and it's one of the more complicated rules... even more than castling it's the rule a lot of people don't know and don't believe exists. 

Works only when the pawn moves two squares to bypass the opposing pawn. Doesn't work with one move forward.

Okey thanks guys! This helps alot!