Jigs can be cast to breaking fish or structure, but most often they're fished vertically. Either the jig is dropped straight down or it's cast a short distance and then allowed to sink to the bottom before the retrieve. Generally you should use the lightest jig that will get to the bottom without excessive delay, although there are a few exceptions. For example, species like snook, striped bass, and grouper often respond to the noisy thump of a heavy jig hitting bottom and the puffs of sand or mud the jig stirs up, while they ignore smaller and lighter jigs. I've used 2- to 3-ounce jigs this way in just a few feet of water with great success.