Sector

A sector is an area referenced by Sidedefs on the Linedefs. Sectors must be closed areas, as in all sidedefs that reference they must make up a closed shape. Note that having unclosed sectors is possible, and can be used for a few special effects, but you shouldn't really attempt to use them unless you're pretty experienced in level editing since they can cause unpredictable results (including crashes).

Ceiling Flat - This is the name of the flat (texture) used to texture the floor of the sector.

Floor Flat - Same as the ceiling flag, only used on the floor.

Ceiling Height - The height of the ceiling.

Floor Height - The height of the floor.

Lighting - How bright the sector is.

Sector Tag - A number used that allows you to alter the sector's ceiling and/or floor height, lighting level or even flats.

Sector Type - A defining property of the sector. These allow you to give a sector some form of damage (for things like lava and slime), have the sectors light level change constantly, enable wind or currents or even push the player in a certain direction.

Sector Structure
For the more technical you may wish to know the actual structure that all sectors have should you decide to make some sort of WAD utility.