OpenGL is a cross-platform, open-standard graphic library for hardware-accelerated rendering. Most Doom source ports which have true 3D rendering use OpenGL. ZDoom itself has two derived ports featuring their own OpenGL renderer, the first of them was ZDoomGL (no longer maintained) and the second is GZDoom, which is considered the current standard for hardware-accelerated ZDoom. ScoreDoom and Zandronum are in turn derived from GZDoom.
OpenGL.org - a hub for OpenGL developers.