I understand a bit about directx features and stuff like HW-TL etc, but I don't get it that well. Could someone please explain to me what exactly are the differences and similarities between opengl and D3D?
They are two different ways of doing the same thing. Basicaly they are two different languages. One owned by MS and rapidly updated. One cross-platform which ends up relying heavily on developer extenstions ('words' made up by a company to make use of thier hardware abilities)