Image stabilization or IS is a Canon technology that optically corrects for camera motion when you take a photo. Since camera motion - caused by handholding the camera, for example - can result in blurring of the image at slower shutter speeds, IS can result in sharper photographs when fast shutter speeds are not possible.

IS is a fairly complex technology involving motion sensors, microcomputer chips and small motors to move key lens elements. There is, therefore, a price premium for IS-capable lenses. But they can be very convenient - when handholding a camera you can easily gain a stop or two over using a non-IS lens.

However, remember that IS does not increase the maximum aperture of the lens or anything. An IS lens with a maximum aperture of 3.5 still has a maximum aperture of 3.5. IS simply lets you use a slower shutter speed than would otherwise be possible when you’re handholding the camera, by compensating for camera motion. So you won’t necessarily be able to get that narrow depth of field that you could with a faster lens - which could be a drawback or a benefit depending on your point of view.

As noted above, Canon EF lenses with AFD (arc form drives) and MM (micromotor) drives use very simple autofocus mechanisms which rely on electric motors and geartrains - rows of tiny cogwheels. Unfortunately, turning such a focus system by hand can damage the geartrain, so such lenses have a switch mechanism which disengages the cogwheels when you focus manually. There is thus no way for you to focus manually when the lens is switched over to autofocus mode.