A lob in tennis (also called Great Horse, in Italian Cavallone) is hitting the ball high and deep into the opponent's court. It can be used as an offensive or defensive weapon depending on the situation.
A lob is usually hit when an opponent is standing near the net, waiting to volley. The ball should sail over the top of the opponent and into the open court behind him or her. A good offensive lob cannot be reached by the opponent, yet it is low enough and has enough pace so that the opponent can not run back and chase it down. It can win the point outright, although with some risks involved.
A defensive lob, on the other hand, simply intends to push the opponent back to the baseline and prolong the point. When a player is pulled out of court, a defensive lob is also useful to buy time to run back and get into a better defensive position.
A lob that is not successful (can be reached by the opponent) is susceptible to being smashed.
In doubles, spin or slice is applied to the lob, so it will go over the head of a player at the net and bounce away from the baseline player.