What is the move called when a pawn reaches the other side of the chess board?

This is called pawn promotion. When the pawn reaches the other side, it can promote to any other piece except pawn or king.

Many players opt to promote their pawn to a queen, as it is the most powerful piece on the board (after the king, of course). However, this isn't always the best strategy; sometimes, a Chess game can be won much faster and easier if the pawn promotes to a, say, knight. It's wise to consider what you're going to do with that promoted piece before you decide what to promote it to.