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 PLY?
PLY, the Stanford triangle format, came out of 3D scanning, and it shows: it carries per-vertex extras like colour and confidence that most mesh formats have no room for. It exists as ASCII and binary; FileSorcery reads the ASCII form.