Many DSLR cameras are APS-C, also called crop frame cameras. This means that they have a sensor smaller than the area of 35mm film.
This difference comes into play when referring to the focal length of lenses. Because an APS-C camera has a smaller sensor, focal lengths have to be altered using a focal length multiplier.
The focal length multiplier varies between manufacturers. Most manufacturers require you to multiply the lens' focal length by x1.6, but Nikon uses x1.5 and Olympus uses x2.
The focal length multiplier is extremely useful to APS-C users when using telephoto lenses, as the focal length will become greater (for example a 200mm Canon lens will be multiplied by 1.6 to become a 320mm lens). However, users lose out at the wide angle lens, usually ending up with standard lenses because of focal length multiplication.
To avoid this problem, many camera manufacturers now produce specific "digital" lenses, which only fit APS-C cameras. These lenses still display regular focal lengths, and they still require focal length multiplication to be applied to them, but they are designed to only cover the area of the sensor used by crop frame cameras.
They are usually a great deal lighter and more compact than normal camera lenses.