What is OBJ?
Wavefront OBJ is the closest thing 3D has to plain text: a list of vertices, one per line, then faces referencing them by index. It has been readable in every 3D tool since 1990, which is why it survives as the format people fall back on when nothing else will open.
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.