FOSS gamedev and creative worlds

I was thinking, what is OpenGL and what it does, can you tell me, (I don't know if I need to tell you this but my OS is Windows XP).
I have Direct3D right now and tell me what the diffrence is.
Thank you if you tell me ;).

OpenGL is like Direct3D, but open source.

OpenGL stands for Open Graphics Library (if you want to learn more, google. :P). Version 1.2 or better is required to run Minetest (pretty much all graphics cards support it, recent graphic cards support 4.2.0 which is the latest version). It is open source, unlike Direct 3D. (and less buggy for Minetest)

Okay but how do I get it? And save it as default because Direct3D is my default.

video_driver = opengl

Add that into the minetest.conf file.

Do I need to download it first?

NO! your video card HAS IT! you'd have to have to have some kind of obscure card from the 90's not to have OPENGL!

Okay thanks for your help guys I now have OpenGL. :)
Credits to: Calinou for telling me the most info for it.
Jordach for telling me to add it to the .conf file
and jordan for telling me that I have it.