This is a Canon Remote Switch (electronic cable release) for cameras with the T3 connection; EOS 600, 620, 630, 650 and RT (these models all require the Grip GR-20, not included), EOS-1, EOS 1N, EOS 1N RS, EOS 5, EOS A2 and A2E, the Canon/Kodak EOS DCS 1 and DCS 3 digital SLRs plus the T50, T70, T80 and T90 manual focus 35mm SLRs.
The cord plugs into the three pin socket provided on these models (a screw in locking collar secures it in place), and allows shutter release from up to 60cm (2 feet) away. This is ideal for any tripod-mounted applications where it reduces vibration, as hand movements aren't transmitted to the camera. The release has a two-stage action, with first pressure activating the camera's metering system, and full pressure firing the shutter. There's also a locking switch for continuous operation.