The chess pieces have been assigned point values to serve as a general guide as to their relative worth.
The standard point values are:
pawn : 1 point
knight : 3 points
bishop : 3 points
rook : 5 points
queen : 9 points
This implies that in general if you trade a knight for a bishop, you don't gain an advantage. You would not want to trade a rook for a bishop, however, since the rook is generally worth more.
Further, it is generally agreed on that two bishops are stronger than 2 knights or 1 knight and 1 bishop.
See the Wikipedia article for different opinions about point values.
Bear in mind, position is everything. That is, there are cases where it's perfectly acceptable to trade a rook for a knight or bishop if the resulting position gives one enough counter-play to make up for the loss of material.
