Camera shake is the thief of sharpness. The tremble of your hand as you hold the camera, the slight jarring when you press the shutter release – even a tiny movement during the exposure can result in blurring in the image.
Much of the time, you won't notice the effects of camera shake. If you're shooting with a fast shutter speed or a wide-angle lens, the blurring may not be significant enough for you to register it – but it will still be there, and it might become noticeable if you make a dramatic crop or a large print of the image.
The obvious way to eliminate movement of the camera during the exposure is to fix it to something that will not move, such as a tripod, and to take precautions against jarring it, such as using a remote shutter release. However, a tripod is effective only if it is sturdy, which usually means heavy, so you can't always carry one with you. There are also many situations where a tripod is just not practical, and several where the use of a tripod is not permitted.
Fortunately, Canon offers another method of reducing, if not eliminating, the effects of camera shake: Image Stabilisation (IS).
There are various kinds of IS. Let's start with lens-based ("optical") IS. The first lens with Image Stabilisation was introduced in 1995. It approached the problem of camera shake laterally. Rather than trying to stop the camera moving, a stabilised lens introduces a compensating movement within it, with the aim of keeping the image static on the camera's sensor.