LIBRARYGerbers and the fab package.

Gerbers are the universal files a factory reads to build your board, one file per copper, mask, and silk layer. Add the drill file, the bill of materials, and the placement file, and you have the complete package a fab needs. Export those, check them in a viewer, and you are ready to order.

What a gerber is

A gerber file describes one layer of the board as vector shapes: where copper is, where the mask opens, where silk prints. There is one gerber per layer, so a two-layer board has a top copper gerber, a bottom copper gerber, two mask gerbers, two silk gerbers, and a board-outline gerber. Gerber is an ASCII format, human-readable and unambiguous, which is why it has been the fabrication backbone for decades (Ucamco).

The modern format

The current format is Gerber X2, which extends the older RS-274X by embedding attributes: which layer each file is, what a pad's function is, even net names. That metadata lets the fab set up your board with less guesswork. When your tool offers X2, use it; a fab that wants plain RS-274X can still read the copper.

The drill file

Holes are not in the copper gerbers. They ship as a separate drill file, historically in Excellon format, listing every hole's position and diameter. Without it the fab knows your copper but not where to drill, so the drill file is as essential as the gerbers themselves (KiCad).

The rest of the package

Two more files turn a bare board into an assembled one. The bill of materials lists every part by reference designator and part number. The placement file, also called the centroid or pick-and-place, gives each part's position and rotation so an assembly machine can place it. Zip the gerbers, drill, BOM, and placement together, and that archive is what you upload.