The jab is a straight punch thrown from your lead hand. There are many uses for the jab, but it's main purpose is to gauge distance and to set up your power punches. You can use your jab to throw your opponent's timing off by intercepting him with a jab as he steps towards you. Or use the jab to stop him while he is in the middle of a combination; a counter jab.
There are many ways to execute a jab. In fact, every trainer I have encountered has a slightly different view of how a jab is thrown. I am not one to hold back anything from you, therefore I will do my best to show you the variations.
REMEMBER: Almost all of the variations will have a corkscrew motion; the forearm twists inward (not the wrist, there are no wrists in boxing). In other words, if you are using your left hand, the motion would be similar to having your palm facing the ceiling, turning it clockwise so that your palm now facing the ground. THIS IS NOT HOW THE PUNCH IS THROWN, just an idea of what the corkscrew motion is.
The first variation is simple. From your boxer's stance, shoot your left arm straight to the target, with a corkscrew motion and return it back to the original position in a straight line. Here comes a list of DO NOTS:
Do not drop your arm. If you drop your arm, you will leave yourself open for a counter.
Do not pull back before punching. If you do, you will give away your intentions. This is known as telegraphing your punch.
Do not throw a wide jab. See above.
A second variation is what I like to call an upper-cut jab. Basically, it's a combination of a wide angle upper-cut combined with a jab. It works especially well if you are in a low stance, knees bent, and then rising up with the upside down jab. There is no corkscrew motion with this punch.
A third variation or way to throw the jab would be to throw it at the stomach. This is a great way to throw your opponent off. Keep him guessing, throw a few punches high, then throw a jab to the body. Be sure you don't drive the jab straight in and don't square up to him, because this will leave yourself open for a downward counter cross. As with all jabs, you are not trying to knock him out with it, just want to stun him and see how conditioned he is. If you noticed that your opponent flinches or squints when you throw the jab to the stomach, then it is a sure bet he hasn't done his abdominal work. Therefore, it's time to drive your hooks and upper-cuts to that area.