I didn't really want this to turn into another Silverlight vs Web technologies war, but I suppose that would be silly of me to consider.
WebGL is a standard defined by the Khronos Group, a very large industry wide consortium (that Microsoft strangely isn't a member of) that defines varies graphics standards including OpenGL. There is also an effort to turn WebGL into a W3C standard as well.
The resemblence to OpenGL is intentional - it allows the same skillsets people have in OpenGL, as well as many of the algorithms to be reused on the web with little to no effort. OpenGL is a standard graphics API to draw 2D and 3D objects onto a screen in a cross-platform manner, it has bindings to a hilarious number of languages and frameworks (including C#). Of course, calling it a "native" API is meaningless.