OpenGL is an API for rendering 2D and 3D graphics. It is part of your graphics card drivers, so where software mode will most likely utilize CPU power, OpenGL (and similarly DirectX) will utilise your graphics card power.

With OpenGL you'll get fancy graphics like dynamic lights, shaders for lighting, glow, fog, etc., as well as texture resizing.

So, if you want the game to look like it originally did, then go with ZDoom as you don't need all the OpenGL options. But if you'd like to have improved lighting or other effects, or you want to play mods that require them, go with GZDoom.

Note that all of the OpenGL features can be turned off in GZDoom, making it identical to ZDoom, should you wish to have different setups but want to use just one source port. (You can store the different settings in separate GZDoom.ini files).

Alright dumb question, I suppose. Will SVE.wad work in gzdoom at this point or no? Because I've seen in the past talk and it ending with 'no this will never happen, go away and leave us alone.'

At this point, it should mostly work, but keep in mind that 1. it will not make use of the enhanced features, and 2. some maps may have bugs because they used the things new to SVE which GZDoom doesn't yet know about. So overall it's better not to use SVE.wad in GZDoom yet.

You can use it with the sve_support.pk3 file from this mod:
http://forum.zdoom.org/viewtopic.php?f=19&t=49836

However, whilst this lets you use SVE.wad, it doesn't bring all of the SVE features over to GZDoom.

Thank you very much for the very clear explanations :)