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 1s linedef is considered void space), and the second is to trigger action specials.

Linedefs can be assigned any of the Action Specials and will trigger them when a specified action occurs. You may flag linedefs to have their special executed when the player walks over the line, when the player uses the line (switches) or when the player pushes the line (similar to Quake or Quake 2 switches). Monsters may also trigger specials by walking over the line, and finally a projectile impact or cross can also trigger a special if you so desire.

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

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.