In fact sweet spot is also known as center of percussion (aka center of the racket face). This spot provides the best “feel” when you hit groundstrokes and also provides the most stability on the shot. The official wikipedia quote: “The sweet spot is a place where a combination of factors result in a maximum response for a given amount of effort. In tennis, squash, racquetball, baseball, cricket, or golf a given swing will result in a more powerful hit if the ball strikes the racket, bat or club on the latter’s sweet spot.
Depending on the size of the strung area (or the head size), the sweet spot area changes (or we could say expands). We can fit rackets in the following categories: