The table saw is often considered to be the heart of most woodworking shops but it can only perform a limited number of cutting operations. The table saw's fence is designed to be parallel to the blade, and the arrangement is well suited for cutting along the long dimension of a board (ripping). The item to be cut is pressed against the fence and fed into the cutting blade.
Without an accessory, however, table saws are not well suited for crosscutting; 'crosscutting' refers to cutting a board along its short dimension, usually against the grain. A crosscut sled is a very useful table saw accessory that expands its capabilities beyond ripping.
As shown in the drawing below, a typical crosscut sled consists of a base board or deck along with front and back fences. Sometimes the back fence is omitted to allow the sled to accommodate larger stock. Beneath the deck, one or two runners are mounted. These runners can be made out of wood or plastic (usually UMHW polyethylene) and they are designed to slide in the slots machined in the top of the table saw. The positioning of the runners and fences are critical to achieve truly perpendicular cuts.
The thick black line in the drawing below represents the kerf created by the table saw blade. Since most crosscut sleds are made specifically for a particular table saw, it is easy to create a zero-clearance deck.
Alternative positioning of the fence on a crosscut sled can be used to make miter cuts. As most woodworkers know, many table saws come with a miter gauge, but they are woefully inadequate.
A carefully made crosscut sled can be a very accurate and indispensable tool in any woodworking shop.