legal move bug

I have question about rules. I don't know if my computer chess game is defective or I have never seen this done but when playing chess on my imac (the chess game came with the computer) i have a white pawn at a2 and one at b3. There is a black pawn at b4. When I move the white pawn from a2 to a4, the black pawn moves to a3 and takes my pawn that was on a4. In other words, the pawns are right beside each other. Is this right. Thanks

Seems when you first learn the game after you finally figure out how the knight moves ,They show you this (en-passant) i just knew i was never going to understand this game. Not to worry soon it will be just another weapon in you arsenal LL