All shaders in TouchDesigner are GLSL shaders.
Materials use TOPs and the GLSL shaders (pixel and vertex shaders) to create materials for 3D geometry.
The most commonly used shader is in the Phong MAT which contains numerous lighting and surface rendering options. The Phong MAT can output the specific GLSL shader code that represents the features being used in the material.
A GLSL TOP uses the GLSL shders for generating images.
See also Write a GLSL Material.