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.
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.