What is difference between Tao and OpenTK

Tao itself contains bindings to alot of APIs, Tao.OpenGL being one of them, while OpenTK focuses on the OpenGL API (and in the future OpenAL).

OpenTK offers bindings to OpenGL too, but additionally alot of utility (Fonts, Vector Math, Enums, etc.) that help you writing more elegant code.

Note that it is possible to use OpenTK for graphics, input and math, and use Tao bindings like Tao.Ode together with it.

Concrete example that accomplish the same task to give a picture:

Tao.OpenGL

float[] v1 = new float[3] {0f, 1f, 2.5f};
Gl.glVertex3fv( (float[]) v1 );

OpenTK.OpenGL

Vector3 v2 = new Vector3(0f, 1f, 2.5f);
GL.Vertex( v2 );