A rapide (quick link) is a lot like a carabiner, only smaller and cheaper. They serve the same function as a rappel ring with the added feature of screwing open. Rapides are used to construct anchors and are often left behind. They are used to connect the rappelling rope to the anchor webbing and provide a durable surface to pull your rope through without wearing through the anchor webbing. They come in a variety of sizes; my favorite is 5/16". They are used in rappelling because they reduce the friction on the rope, so it's easier to pull the rope from the bottom. Small rapides are good for setting knot blocks or biner blocks.