is basically ZDoom
but with an additional renderer which uses OpenGL
. Every feature that is in ZDoom is found in GZDoom (though you can have to wait a bit for the latest ZDoom SVN
novelty to be added in the GZDoom SVN; but not vice-versa. Independently of the renderer, the physics code is slightly different (to account for 3D floors; they're invisible in the software renderer but still present anyway, while in ZDoom they're completely ignored).
What does "hardware" and "software" mean? Simple. With the software renderer, Doom makes all the graphic work itself (so the computations are handled by the CPU). The hardware renderer, however, calls upon a graphic library (Open GL == Open Graphic Library) that delegates most of the work to your graphic chipset and its GPU, which has specialized components to do standard graphical operations faster than normal code does (hence, hardware). Unless you have a very powerful CPU coupled with a pathetic GPU, the hardware renderer is going to be much faster. The downside is that, since it's much faster, you can ask it to do much more (such as dynamic lights); and overdoing that can result in a slower render at the end.