Most Gem objects are wrappers to OpenGL commands.
While pd-list is indeed full of helpful fellows, I would suggest looking into documents on OpenGL for facts and design concepts unclear through reading only ones installed with Gem.
i.e., as I see from searching, your question may be new to this list but appears to be an FAQ in the OpenGL field.
Although I do not recommend it, flipping through the first 4 chapters of the "OpenGL programming guide" http://www.opengl.org/documentation/red_book/ even if you are not specifically a C programmer.
It helped me catch up with sooo many WHYs of Gem which had me puzzled for years.