Any eyeglass that focuses at around 28 inches can be used for the computer, but only optical quality eyeglasses specifically designed for computer use are referred to as computer glasses.
Both trifocals and weak bifocals can be used but with limitations. Since the computer portion of these glasses is at the bottom of the lens, and most moniters are at eye level, the head must be tilted back to see thru the computer portion. Since this is not a natural head position, it only takes a few minutes for the neck muscles to tighten up and cause a stiff or sore neck.
Computer glasses are more like reading glasses in that the whole lens focuses at the same distance and does not require you to tilt back your head to see the computer screen properly.
Unfortunately most reading glasses don't work well for the computer because they focus at around 14 inches, and the computer is nearly twice that distance from the eyes causing the computer screen to be out of focus. To bring the monitor into focus you would have to lean forward. This unusual position usually causes the back muscles to tighten up resulting in a sore back shortly thereafter.
Rather than lean forward any longer some will lean back to relieve the strain in their back. Since the screen is now out of focus they will squint their eyes in an attempt to see better. This squinting causes pressure and tightness around the eyes which can lead to eye strain headaches and wrinkles at the outer corner of the eyes call crows feet.
The solution is to use special computer glasses which look just like regular glasses. If you already wear regular distance glasses, you can use clip on computer glasses to wear over your regular glasses when using the computer.