Chess - Learning to Sacrifice Pieces for the Greater Good
A sacrifice made at the beginning, with no clear and immediate advantage, is called a gambit.

There is no hard "rule" for when a gambit is worthwhile or not; it all comes down to what imbalances it gives you, and whether you think you can use those to your advantage.

For example, in the Danish Gambit (accepted), white sacrifices two pawns, hoping the extra space/development he gains will help mount an early attack.  In the Queen's Gambit (accepted), on the other hand, white sacrifices the pawn knowing he will either get it back, or black will have to severely weaken his position to hold onto it.