If we use the Wiki definition, then
In the game of chess, a sacrifice is a move giving up a piece or pawn in the hopes of gaining tactical or positional compensation in other forms.
How much compensation is needed to make the sacrifice worthwhile depends on just how much material we have given up. If we give up a pawn only, then improvement of position or the gaining of helpful tempi (moves) may be enough. If we're giving up more, then the threats generated or opportunities created had better be significant. For example, if we give up a rook for a knight, but our opponent will spend the next 10 moves trying to stave off mate, it might be a worthwhile sacrifice. Here, we'd be giving up material for extreme pressure and tactical opportunity.
Some sacrifices are classic and have been around a long time. Some are opportunistic and simply appear out of a conflux of conditions in a middle game never explored before.
Sacrifices are always exciting - especially if we're the ones pulling them off and actually realizing benefit.
Hope that helps a little.
helps with the concept of why they occur in the first place
i have noticed in just about every good and gm game, sacrifices lead to pawn rook, and even in some really great games like alapin or alapans(sorry i forget minute details like that) he sacrificed his queen gladly for a rook which is nuts! maybe its because im still only a 1000 rating player. thanks in advance if anyone can offer advice