Portal:OpenGL Shading Language/Tessellation Evaluation Shader
The Tessellation Evaluation Shader (TES) is the shader stage responsible for computing the actual tessellated vertices. The fixed-function tessellator takes a patch and breaks it into pieces. The TES takes those smaller pieces and figures out how to interpolate the original patch data to generate the new per-vertex data for each tessellated vertex.
The TES operates similarly to a vertex shader, in that there is a 1:1 correlation between a vertex in the tessellated patch and a TES invocation. Thus, every TES is responsible for computing a given vertex in the tessellated patch.
The TES is mandatory for tessellation. If there is no active TES stage, then no tessellation will happen.