A Spinnaker is a large, bulbous foresail that is used for downwind sailing. Spinnakers are usually made of very light material and resemble a parachute when deployed; sometimes a spinnaker is referred to as a "chute". Spinnakers often have colorful designs on them.
Spinnakers come in two varieties: symmetric and asymmetric.
A Symmetric spinnaker resembles a tall isosceles triangles. Either lower corner can be either a tack or a clew. It must be deployed with a spinnaker pole. The spinnaker pole is attached to the mast at one end, it uses a topping lift to support its weight, and it often uses a foreguy to help control its vertical position -- it should be kept parallel to the water. The spinnaker pole is deployed to the windward side of the boat (opposite from the mainsail). A sheet runs to the leward corner of the sail, and an afterguy runs to the pole to control its horizontal position.
Asymmetric spinnakers (or A-sails) are more like very large genoas. One corner is the tack, which is attached at the bow of the boat. Sometimes a tack line is tied to the corner of the sail and runs through a block at the bow of the boat to allow the sail to rise up above the level of the deck. The other corner is the clew which is attached to a sheet. Asymmetric spinnakers do not use a pole.