EF — This refers to Canon's electronic lens mount, used in all Canon camera since 1987. The previous mechanical lens mount carried the letters FD.
28 - 70mm — this is the range of focal lengths covered by the lens, 28mm at the wide end and 70mm at the long end (for example).
f/3.5-5.6 — this is the range of the maximum aperture of the lens, in this example f/3.5 at the lens' wide angle setting, changing to f/5.6 at its longest focal length. A prime or zoom lens with a fixed maximum aperture will have just a single F stop shown in the name.
L — Canon's designation for their top-of-the-line lenses is the letter L. These lenses are the best that the company knows how to make at any given focal length and aperture, and of course for a given price (which in the case of L series lenses is usually high).
DO — stands for Diffractive Optic and is used on Canon lenses which utilize a unique diffraction grating to control chromatic aberration and reduce lens size and weight.
IS — indicates that the lens utilizes Image Stabilization, which reduces vibration as much as 3 stops, allowing hand holding of long lenses at much lower shutter speeds than would otherwise be possible.
USM — stands for Ultrasonic Motor, a technology that uses sound waves to position the lens elements when focusing. This reduces size, weight and power consumption of the autofocus mechanism as well as increasing speed and allowing for manual override.