The jab is a quick, straight punch thrown with the lead hand. Often the most important punch in a boxer's arsenal because of its power and its ability to set up other punches. It can also be used as a way to gauge distance, to keep an opponent wary, or as a defensive move to slow an advancing opponent.
Note that the straight left differs slightly from the left jab. With the jab, the elbow should be pointing at the floor at full extension. A straight left turns the elbow upward. (While the hook has the elbow parallel with floor, with the arm bent.)