Jigs are a staple in bass fishing. They are usually large hooks with colored strings,called skirts, usually made out of rubber.
Trailers, made out of soft plastics, are attatched onto the back of jigs to make them more attractive to fish. For more on soft plastics, look here
Here's an image of a jig, with blue and black coloring
Good time to use Jigs
In addition to normal jigs, bladed jigs are also popular.The blade helps create a disturance in the water, making it better for darker, muddier water. This helps fishermen get more reaction strikes. For more on the behavior of fish, check out Behavior of Fish