A carabiner or karabiner is a gated connector: a metal loop with a usually sprung gate designed as a connector that can quickly join together rope and chain. There are two basic types of carabiner; the climbing carabiner designed to connect ropes used in climbing, and the smaller chain clip which is designed for joining chains. Both types are a must have in any Toy box as they provide a quick and versitile way of attaching chains to bondage cuffs and other SM equipment.
A word of warning though, they can be difficult to unclip if the chain they are attached to is under tension or the binder's hands are oily. Also, if there is the risk of the bound person fainting and hanging from their wrists then carabiners should not be used as they would be almost impossible to unclip. In these cases, panic snaps should be used.