Vignetting is simply that the corners or edges of your image are darker than the center. It is caused by the lens, but can also be created artificially in post processing software. The term light falloff is also used for the same.
Some use vignetting to draw attention to the center of the image and hence find it useful.
Most lenses exhibit some level of vignetting and it is most common for lenses that can open up and let in a lot of light (say f/1.4 or f/1.2). Some cameras are able to correct the vignetting as part of the cameras processing of the image, so it is never visible to the photographer.
