The tennis statistics at the end of a set always show the number of winners and unforced errors of the players. But when does a point count as a winner and when as unforced error?
Winner - a shot hit by a player so the ball jumps more than once on the opponent side without the opponent being able to even touch the ball.
Unforced error - a mistake made by the player from quite an easy shot hitted by from the opponent for example hitting the ball wrong, lack of precision or improper position.
Number of unforced errors usually counted for statistics and indicate the level of each player's game.
(quick addition to Dor Cohen's great explaination)/ people checking this question might be interested in:
Sometimes you will see forced error statistics, which are generally calculated when an opponent hits a great shot which is very difficult to get back, and the person returning the ball misses.