What is OFF?
OFF (Object File Format) is about as small as a mesh format gets - a header, a count of vertices and faces, then the numbers. It turns up mostly in academic geometry work and mesh-processing datasets, where being trivial to parse matters more than being widely supported.
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.