I'm starting to learn about 3D rendering and I've been making good progress. I've picked up a lot regarding matrices and the general operations that can be performed on them.

One thing I'm still not quite following is OpenGL's use of matrices. I see this (and things like it) quite a lot:

x y z n
-------
1 0 0 0
0 1 0 0
0 0 1 0
0 0 0 1

So my best understanding, is that it is a normalized (no magnitude) 4 dimensional, column-major matrix. Also that this matrix in particular is called the "identity matrix".

Some questions:

• What is the "nth" dimension?
• How and when are these applied?

My biggest confusion arises from how OpenGL makes use of this kind of data.