A flat is an image that is drawn on the floors and ceilings of sectors. Flats are very different from wall textures. Flats are a raw collection of pixel values with no offset or other dimension information; each flat is a named lump of 4096 bytes representing a 64×64 square.
Flats are always drawn aligned to a fixed grid. This ensures that floor and ceiling textures flow smoothly from sector to sector. It can also cause problems for level designers, usually when placing teleport pads.
Certain flats are animated to represent water, lava, blood, slime, or other substances. See animated flat.
Complete details are in the Unofficial Doom Specs.