The word pannier is derived from the Latin word meaning bread basket, but these days cyclists stash all sorts of things in bags designed specifically for cycling that bears the same name. Usually attached around the back tire of the bike, panniers are great for carrying the commuter's briefcase or groceries for a car-free shopper. If you're interested in touring with your bike, panniers are a must. You can find sets for the front tires as well as back, so you can haul all your camping equipment.
The pannier pictured is from the Dutch company Basil, but panniers come in more serious looks too. REI has a great selection, from waterproof bags to panniers that slide off the bike and transform into an easy-to-carry messenger bags.