A spinner is a time-tested and very effective type of freshwater fishing lure. It typically consists of a swiveling side-mounted spoon at the front of the lure, a weighted midsection, and a treble hook dangling from the back of the weight. Very often the hook itself will be dressed with some kind of hair or feathers to disguise it. Spinners come in many different sizes, colors, and dressings.
Spinners are not usually designed to look like anything per se, but when fished normally they probably imitate a minnow. When pulled through the water at almost any speed, the spoon attached to the front of the weight will spin quickly around the lure like a helicopter blade, hence the name. This creates a disturbance in the water that attract the fishes' attention. For finicky fish, an over-regular vibration might seem suspicious, especially if it resembles a boat propellor. For this reason, spinners are best fished by reeling in slowly, then periodically jerking them forward with the rod tip. This can help simulate the action of a wounded baitfish.
In addition to straight cast-and-retrieve, spinners are also excellent for slow trolling, bottom-fishing, and even jigging. When trolling, vary the action on the spinner by dipping your rod tip to slow the lure, then pulling up on it to jerk it forward. Spinners can be fished effectively on the bottom by letting them settle, then reeling in for a bit; lather, rinse, repeat. Spinners are usually more snag-resistant than other lures, because the spinning blade will cause the lure to bounce off potential snags. However, it can often get tangled in thin weeds.
Many kinds of spinners are available wherever freshwater fishing equipment is sold. The most famous and widely trusted brand is Mepps. The Mepps Aglia, in particular, is touted as "The World's #1 Fishing Lure", and is an essential part of any freshwater angler's tackle box. Be wary of cheap, generic brand spinners, as they can often have very poor action due to badly-engineered blades.