They can only capture other pieces one square diagonally. The Pawn is the weakest piece on the board and is worth 1 point. Knights and Bishops are equal on the board. They are worth 3 points each. You could capture 3 pawns for about 1 Knight or Bishop it might be considered a fair trade.
Rooks are worth five points.
If you lose a Rook for his Knight it is NOT considered a fair trade. Rooks can be powerful if you know how to use them. The Queen is the most powerful piece on the board. It is worth nine points. You would win a deal to trade two Rooks for a Queen.