Common abbreviation of wideangle lens. Normally opposed to telephoto lens. It is a lens whose angle of view is bigger than the angle of view of a normal lens (which is approximately 45 degrees).
In 35mm cameras, a wideangle lens is anything that is 35mm of focal distance *or focal length)and below. As the focal distance gets shorter, the angle of view gets wider.
As they get wider and wider, wideangles have increasing problems with: Basically, it is difficult to build a lens that is very wide and still provides an image circle big enough to cover a 35mm lens: people stop trying AFAIK at 14mm. Anything shorter than that usually does not fill the photogram.
My view of practical focal lengths in wideangles for the 35mm format:
  • 35mm: everyday workhorse, so much that it is the lens installed on most of the fixed focal length compact camera.
  • 28mm: this could be your first wideangle lens. It appears in every camera maker's catalog, and can generally be bought quite cheap.
  • 24mm: starts being difficult to handle.
  • 20mm: the angle of view is 94 degrees, which makes it great for taking pictures in cramped indoors, or for great, scenic landscapes. It is a bit difficult to handle, though, because such a wide angle means that it will pull in a lot of crap, like uninteresting skies, people you don't want to photograph ...
  • 16mm and below: specialized lenses, quite expensive usually, very prone to flare because of the inevitable protruding front element.

One interesting propery of wideangles is their depth of field, that increases as the focal distance goes down: this means that with a wideangle it is quite easy to have a lot of the scene (or practically all of it) in focus.
For example, a properly focused 20mm lens at f11 renders everything from about 0.5 meters to infinity as sharp (of course, there are different definitions of sharp, but let us just say "sharp enough").
Great depth of field is neither good nor bad, it is simply a tool for the photographer.
One typical application is near-far shoots where you have a distinctive subject in the foreground, and an interesting background that stays sharp all the way to the faraway mountains and majestic peaks.

Wideangles, especially extreme ones, are not well suited for taking pictures of people, unless you want to distort their features or, to be more precise, to surprise the viewer with a novel perspective, which may not be well received.

Another interesting property of wideangles is that, again due to geometry, they reduce the minimum handholding speed, that is to say, you can use them with a longer exposure time without getting "moved" pictures.
Normally, one reccomends an exposure time equal to 1/focal_distance, that's to say that a 24mm lens will gain one full stop over a 50mm --- a point somewhat moot to me, because anyway a 50mm will be typically brigter than a 24mm.

HAPPY HAPPY JOY JOY !!!: please do read the below writeup by Wicker Nipple: it clarifies many things.