Image Stabilization (IS)
Nearly all cameras, including many with powerful lenses, now come with an image stabilizer, a device that compensates for handheld camera shake. Often, the IS device lets you shoot with a slower shutter speed than you otherwise could without producing blur due to hand shake (although it won't compensate for a subject's motion). Optical (in the lens) and mechanical (in the camera body) image stabilizers are the best types to use, although some cameras include simulated stabilization.
In digital SLRs, some brands include mechanical stabilizers, which can use IS with every lens. However, some SLR brands only include optical IS in telephoto or long zoom lenses, the ones that need it most. The optical-based IS generally produces better results than mechanical-based IS. Note that you will not find IS on every lens as it is not built into the camera body. If your camera has an optical zoom greater than 3x, then image stabilization is a must feature.