The OpenGL Shading Language (GLSL) is the principal shading language for OpenGL.
While, thanks to OpenGL Extensions, there are several shading languages available for use in OpenGL, GLSL (and SPIR-V) are supported directly by OpenGL without extensions.
GLSL is a C-style language.
Compilation model
GLSL is quite unique among shading languages due to its compilation model. It’s compilation model is more like the standard C paradigm. Compilation is overseen by a number of object types. Note that these do not follow the standard OpenGL Objects paradigm.