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.
Floors and ceilings can be animated to represent water, lava, blood, slime, etc. A certain few flat name sequences are built into the doom engine. Floors and ceilings defined to use a flat name from one of these sequences will rotate through the entire sequence, for example NUKAGE1-NUKAGE2-NUKAGE3.
Placing flats in PWAD files presents a number of difficulties.
Complete details are in the Unofficial Doom Specs.