Linedef

Linedefs are what make up the 'shape' (for lack of a better word) of your map. Every linedef is between two vertices and contains one or two sidedefs (which contain texture data). There are two major purposes of linedefs. The first is to give your map a definitive boundary between the player and the void (any area of a map behind a one-sided linedef is considered void space), and the second is to trigger action specials.

There are three ways to trigger an action: (Which of these applies depends on the line type number.)
 * walkover
 * "use" (with the space bar, by default)
 * shoot

The categories of line types include the following:
 * open and/or close doors (local and remote)
 * move ceilings and floors up or down
 * start and stop moving floors
 * start and stop crushing ceilings
 * activate lifts
 * build stairs
 * change light level
 * teleport
 * special effects (such as a scrolling wall)
 * exit level

The specified action will take place in the sector or sectors that have the same tag number as the linedef. Exceptions to this include local doors (which act on the sector on the other side of the line), special effects that apply to the linedef itself, and exit level.

Most actions can be specified as either once-only or repeating (by using different line types).

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

Linedef Flags
Linedefs also contain a two-byte (16 bits) field reserved for various flags. Flags are as follows: