STL: STL is a 3D file format that describes the surface of a part as a list of triangles, with no units, colour or material, and is the most common file sent for 3D printing.
STL dates from the late 1980s and the first stereolithography machines. It stores one thing: a set of triangles that together approximate the outside of a part. Every CAD program can export it and every slicer can read it, which is why it is still the default file for 3D printing.
That simplicity has costs. An STL has no units, so a file drawn in inches and one drawn in millimetres look the same to software (see file units). It has no colour, material or assembly information. Curved surfaces become flat facets, so a fine export can be very large and a coarse one visibly faceted. And because it is just a list of triangles, it can contain holes, overlaps and flipped faces that stop it being a watertight mesh.
Example: a 50 mm sphere exported coarsely might have a few hundred triangles and look like a golf ball; exported finely it might have hundreds of thousands, and a file size of tens of megabytes.
STL comes in two flavours, ASCII (text) and binary. Binary is far smaller and is what most tools write by default.
For machining, an STL is a poor choice: a mesh loses the exact geometry a CAM program needs, so STEP is preferred. For printing, 3MF fixes most of STL’s gaps. Read STL vs STEP vs 3MF for a side-by-side.
Go deeper: STL vs STEP vs 3MF (and OBJ, DXF, IGES): which file to send for a quote