I don't understand this move

Chess noob here, confused by a move in the game I'm playing.

I've not seen that happen before. What's the rule?

It's the en passant rule.  It's all here.

http://en.wikipedia.org/wiki/En_passant

Thank you! I was scratching my head for a while there.

Are there any other unusual rules? Wiki lists castling, en passant, and pawn promotion, which I now know all about, I think. Any other rules to know about?

Theres also:
- If you run out of time but your opponent does not have enough material to deliver checkmate (or a pawn to promote to queen) it is a draw.

-Stalemate confuses some people

-some rules not applicable online such as touch move and touch take.

-3 move repetition and 50 move draw (no captures and no pawns have been moved in this time)

You should read this short article: http://www.chess.com/learn-how-to-play-chess.

Thanks, this all helps very much.