opengl, opengl shading language, cg-shader, sgi-shader in a context

Hi @all,
I´m a complete Newbie. I´m just starting with the topic of grafics programming. I´ve allready read a lot of stuff but I`m still a little bit confused about the differences and dependencies between OpenGL, the OpenGL Shading Language, Cg-Shader and SGI-Shader.
Could anybody explain it to me in simple words, because my second problem is that my english isn´t very well, so that I´m not sure about the things I read.
Thanx a lot.

You might try these links (as a start):