What is STL?
STL is the language of 3D printing. It describes a surface as nothing but a heap of triangles - no colour, no texture, no structure - which is exactly as much as a slicer needs. It comes in an ASCII flavour and a binary one roughly a fifth the size.
What is GLB?
GLB is glTF packed into a single binary file - geometry, materials, textures and a node hierarchy in one blob. It is the format the web actually uses for 3D, and the one most viewers, AR tools and game engines want.