Washers have a number of different uses, usually being used along with nuts and bolts. Sometimes they are used for making an even surface for a nut or bolt, while at other times they just serve as spacers.
Flat washers, like those in the assortment at the right, are the most common and well-known.
Lock washers assist nuts by keeping them held in place with tension and/or friction. There are a variety of designs available, some of which work better in certain applications.
The basic lock washer seen above to the left has a spring-like design which grabs on to a nut with tension. At the right is an external lock washer, which uses the grip of the teeth to grab the nut, and in some designs, the other surface the washer makes contact with. The angle of the teeth allows surfaces to slide against it with ease when turning clockwise direction, while resisting any counter-clockwise motion. Internal tooth lock washers use the same locking method, but have an internal design for smaller surface areas and nuts.