A: The lob is an effective shot in tennis. What is it? It's a ball where you're trying to hit the ball substantially high in the air perhaps over a net player's head or perhaps just to make someone in the back court have a difficult time with a high bouncing ball.
Generally there are two types of lobs, defensive lobs and offensive lobs. A defensive lob you would hit when you're chased off the court and your opponent perhaps has come forward and you're in trouble. The logic is send it high, buy some time to get back in the court, and try to reset yourself.
The offensive lob is a whole different animal. Someone's come to the net against you. He thinks you're going to hit a driving passing shot, and you cross him up by brushing up a lob hitting it over his head. If that ball gets over his head he's done. He's done because an offensive lob is hit with top spin where you create a tremendous amount of rotation on the ball. You hit it very high over your opponent's head. And when it bounces it's ungettable, because when it bounces it takes off.
A defensive lob doesn't have top spin. It's simply a desperation shot that you try to get over your opponent's head, so that it doesn't have any spin but you have a lot of control over it. You hit it very high, but when it bounces it takes a very lazy bounce. It bounces almost straight up. So, often a net man can run back and get a defensive lob after it bounces. But the key is to try to make them run back and hit a ball when they're running backwards rather than having them hit an overhead.
So, a lob you want to get over your opponent's head. Think twice about hitting lobs from the middle of the court. If you happen to hit a poor lob you might want to check how paid up your life insurance is. So, usually you'll use the lob when you're behind the baseline and you have a little time to handle a well hit overhead.
B: I tell my students, I rarely use superlatives, but I tell them never lob a short ball unless your opponent is basically draped over the net. Because it's very hard to control a lob from a short position. You basically want to be behind the baseline when you execute a lob. That's what a lob is all about.