Well this is a great place to start. Basically a 3D accelerator is a graphics card which can draw polygons, using only the polygons 3-dimensional vertices, and which can map textures on polygons and/or shade polygons.

Not all graphics accelerators are 3D card. Up until 1997 most graphics accelerators were just 2D- they drew two dimensional images by spitting out pixels onto to the screen without calculating that hidden element- the z-axis.