Due to it's relatively low value, in chess lingo, the pawn is not referred to as a piece alongside the bishop, rook, queen, king and knight. It is just the pawn. "Piece" or not, when used in concert with each other, pawns can be a formidable force. Since no one wants to exchange a piece for a pawn, pieces are often forced to move around and away from pawns in the opening and middle game, allowing the pawns to dictate the layout of the board. It is for this reason they are famously referred to as "the soul of chess".
From their starting position the pawn can move forward either one or two squares. Afterwards, they are limited to moving only one square forward at a time unless they are capturing, then they move diagonally. The pawn moves forward but kills diagonally. The pawn is also capable of one of chess's most disputed moves, the en passant (please see the en passant node for all the gritty details of this special move). Finally, should your pawn make it to your opponent's back rank you are able to turn it into any piece (except a king) that you wish. For more pawnly goodness see: Pawn Structure, Passed Pawns, Double Pawns, Isolated Pawns.