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.

Every conversion below runs in this page, on your own machine. Nothing is uploaded, there is no queue, and there is no account to make. Pick a target format, or just drop a STL file above and choose afterwards.