Lightmaps are a nice way of getting around the backdraws of per vertex lighting without the cost of per pixel lighting. Basically a lightmap is a map of the color value at certain points of a face that is to be rendered. A lightmap is computed ahead of starting to render a level: You put a more or less coarse grid over each face and compute the color value for each grid node. The coarser the grid, the faster the process, but the edgier the resulting lighting looks. The good message about this is that the lightmap doesn't have to be overly detailed: OpenGL's color interpolation will do the rest. The following threeimages show lightmaps at 8x8, 32x32 and 128x128 pixels. While the last lightmap looks finer if rendered by itself, you won't see a difference when lightmaps are rendered in conjunction with textures.

The best thing about lightmaps is that they work perfectly together with per pixel lighting for dynamic light sources and can greatly relieve the burden put on the hardware for computing per pixel lighting."