A spoon is a simple, yet effective and versatile variety of fishing lure. It is shaped like its namesake, minus the handle, and in fact I've seen handmade lures made from teaspoons. Typically, it has a hole on the front end for attaching your line, and a three-pronged treble hook dangling from the back. Some varieties have a single hook soldered to the concave side of the lure instead. Spoons come in many shape variations, from thin and almost flat, to thick and narrow and wavy. Any color may be used, although the standard is just a polished silvery finish. Any lure that is simply a piece of metal with a hook, shaped to move in the water, may be called a spoon.
A standard spoon's action in the water roughly imitates a minnow or other small baitfish, and the flashing metallic glint adds to this. As the spoon is pulled slowly, it wobbles back and forth with little resistance. As it is pulled faster, it can spin completely around the central axis with a great deal of resistance. You should be able to feel the difference through the pole. Either action can be effective; as with most lures, a faster, more violent action will attract aggressive fish, but may scare away more elusive or shy fish. If you don't specifically know the nature of your prey, it is probably a good idea to vary the action; for example a cycle of 5 seconds of fast action and 5 seconds of slow action. Spoons can also be good for jigging, especially the thin kind that flutters slowly on the way down.
One very common use of spoons is as a supplement to natural or artificial bait, or other equipment like soft plastic worms. The single-hooked variety of spoon is particularly well-suited to this, dangling the bait behind and underneath on the hook. A weedless spoon (single-hooked with a spring against the hook to brush aside weeds), fitted with some kind of smelly, artificial bait, can be extremely effective for catching gamefish that live in weedy areas, like largemouth bass. The attachment of extra equipment to a spoon will usually deaden its movement, but many baits like rubber worms or rubber grubs have swirly action of their own. The shiny spoon can help grab the visual attention of a fish when the plain bait might not.
You can find many kinds of spoons at any sport shop that carries fishing lures and other angling equipment. Time tested varieties of spoons are the red-and-white-striped Red Devil, and the Johnson Silver Minnow, which is an of the weedless single-hooked variety. Or, you can make your own with a few tools and some silverware. I once made several spoons from common american pennies, by hammering it into a rounded shape over a rock, drilling holes in either end, and attaching a treble hook with a pop swivel. As it turned out, it had a rather unique action that was very popular with panfish and small gamefish like perch and crappie. I also made a double-jointed version by attaching two pennies with a swivel, for larger fish.
BlueDragon says re spoon: Mackerel positively throw themselves onto the hook if there's a spoon :)