Noclip is a command that prevents the character from colliding with other objects and permits the camera to move in any direction; allowing it to go through such things as walls, props, and other players in a movement similar to the fly cheat. The player can travel into vacuums or voids, undeveloped areas that have nothing in them. As cheats are commonly disabled in multiplayer games, Noclip is primarily used for levels intended for Single Player use.
In Quake, the player used the swim up and down keys to navigate Noclip mode. In Quake II, the free look system enabled the to go anywhere easily without the need to press two separate keys to fly unlike in Quake. However, Quake's Noclip can perform like Quake II's if used in conjunction with the mouselook cheat.
In Quake III Arena it is harder to activate Noclip mode and other cheats than in previous games. To be able to use codes in Single Player mode, in the Console, type: /devmap <name of map>, Ex. q3dm11 for cheats enabled for that level. Then the level will load with cheats enabled. Enter /noclip and /god if desired.
In Quake 4 Noclip is also available. However, enemies can see you now whereas in Quake II the player was invisible to enemies. The only solution is to activate "godmode" to not be killed or damaged.
Noclip can be used to access areas the player wouldn't normally be allowed to access by normal gameplay, I.E: Going to an area quicker, skipping boss battles, etc. It is normally considered a cheat due to these implications, however it can also be used to bypass glitches.
Noclip is an important tool for level designers as it allows them to bypass certain parts of the level. This allows them to see how other areas they may be working on at the time are coming along, without having to play up to that point every time they test a large level. Examples would include "testing" the difficulty of specific rooms, adequacy of item versus enemy placement, ensuring a scripted event works properly (such as activated switches, trains, enemy spawns) and testing other level obstacles, such as jumps or crushers, to see if they are functional and/or appropriately difficult. This also is useful to navigate a level without collecting pickups when attempting to take a screenshot for the loading screen.