What is OFF?

Object File Format, and about as plain as a mesh can get: a count of vertices and faces, then the vertices and faces. It comes out of academic geometry work, where nobody wanted to parse anything more complicated to test an algorithm.

What is STL?

The language of 3D printing. It describes a surface as a heap of triangles and nothing else - no colour, no texture, no structure - which is exactly as much as a slicer needs. There's an ASCII flavour and a binary one about a fifth the size.