This section covers general 3D programming, but some discussion of different things is in order.
OpenGL is a standard for displaying graphics. Without getting into PC versus Workstation politics (ancient history now), it was a standard that a workstation vendor (SGI) pioneered to standardize computer graphics and the ability for programs to display graphics.
There are other APIs, such as DirectX, which was backed by Microsoft and many PC game developers, CAD software, and other PC computer graphics.
OpenGL is not, strictly speaking, open source; yet the software can be used without royalty payments and is documented and freely available (to be fair, so is DirectX).
Basically, OpenGL is a way for software to display graphics. Software, in this case, usually means C++ (or other languages that can call native libraries and O/S utilities...
