OpenGL ES 2.0 combines a version of the OpenGL Shading Language for programming vertex and fragment shaders that has been adapted for embedded platforms, together with a streamlined API from OpenGL ES 1.1 that has removed any fixed functionality that can be easily replaced by shader programs, to minimize the cost and power consumption of advanced programmable graphics subsystems.
OpenGL 2.0 as a reference - the specification is defined relative to the OpenGL 2.0 specification.
OpenGL ES Shading Language - adds the same basic shading language as used in OpenGL 2.0, but adapted for embedded platforms.
Programmable Pipeline of OpenGL 2.0 replaces the fixed function transformation and fragment pipeline of OpenGL 1.x. (see diagram below). Using shaders minimize the cost and power of advanced programmable graphics subsystems.
I hope you can get an idea...