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. The pixel values are converted to actual colors in the same way as for the Doom picture format.
Flats are always drawn aligned to a fixed grid. This ensures that floor and ceiling textures flow smoothly from sector to sector.
Certain flats are animated to represent water, lava, blood, slime, etc. See animated flat.
Placing flats in PWAD files presents a number of difficulties. In particular, all flats used (not just those replaced) must be present in the PWAD.
Complete details are in the Unofficial Doom Specs.