OpenGL is a desktop computer centric API (like Direct3D). WebGL is derived from OpenGL ES 2.0 (intended for mobile devices) which has less capabilities and is simpler to use.
WebGL is also designed to run in a browser, and has therefore a few limitations more then OpenGL ES 2.0.
Unlike OpenGL, WebGL does not require native driver support. A wrapper called “angle” which Safari, Chrome and Firefox use can translate WebGL calls and GLSL to either OpenGL/GLSL or Direct3D/HLSL
Please see the khronos wiki for a detailed answer: http://www.khronos.org/webgl/wiki/WebGL_and_OpenGL_Differences