Map editors quite simply allow the user to edit maps. This statement encompasses more than it might initially seem. There are many different methodologies to editing maps and creating maps, some of which will be addressed later. The key difference, as mentioned above, between and in-game map editor and a map editor outside of the game is the usability. Hence the name, the in-game map editors will be included on disk or with whatever package the game ships, thereby increasing distribution off the bat. More importantly in-game map editors are designed for the average game player to be able to manipulate and utilize so there is a much smaller learning curve than those map editors out of game that usually involve some form of programming.
Typically map editors will offer players the suite of objects, weapons, and vehicles that are included in the game to be used in whatever methods they desire. The player from there must have some concept of the map they wish to make, and some idea of how that map is to look. Before this can happen the player must analyze the tools that the in game map editor offers.
Often, in an effort to reduce complexity, map makers will limit that amount of actual creation that the player will have to do. The easiest way to see this is the actual object palette that the player has to work with. Typically in game map editors will not offer the player the ability to design new objects, weapons or vehicles in order to reduce the complexity and preserve the integrity of the original game.
Other limits that arise typically stem from the ability to manipulate the actual geometry of the levels. If we look for example at Halo 3's Forge map editor, the player is unable to edit any of the existing geometry on the maps, and must make due with the provided objects to create new works. Other map editors, such as Far Cry 2's, give the player much more freedom in allowing them to have free control over the terrain and to create different environments ranging from sparse deserts to dense jungles to crowded cities packed with objects.
In game map editors will usually give the player the ability to manipulate the map in all three dimensions. This can lead to entirely new styles of play and new styles of map design. These tools will allow the map makers to create structures ranging from floating castles to bottomless pits, and can increase the variety of the maps.
Every map starts with an idea for how the player wants to play something. This typically is one of the predesigned game types that the game ships with, however what is becoming increasingly popular is to have players design entirely new game varients based around the previous gametypes and the map structure, but that is another topic. So after the player has decided on what game type they intend to suit they need to come up with a style of play that the map will suit.
Maps can be generalized to fitting gametypes such as deathmatch, capture the flag, assault, or conquest/territories for the most part. Beyond that maps often have their own unique flavor of these gametypes and no map plays exactly the same as any other map. This is usually because each gametype centers around key points on the map that the opposing teams will vie to control, be it the flag spot. territory, or a strategically placed room. To design a map, a player must take into account what they believe these key points will be and find some way to make they unique.
After an idea has been settled on and the key points mapped out, then the actual construction of the map must begin. This is the point at which the player will find they might deviate from their original goals because of spontaneous ideas that arise. It is important to keep in mind through this process that to make a map too clustered with little things will ruin the gameplay experience and dilute the true purpose.
When the physical construction of the map is complete it is then time to place the objects, weapons and vehicles on the map. Typically objects will add to the environment, so if the map is themed off a specific place these should be used to enhance the experience. Objects can also provide other uses such as cover, so that must be taken into account as well.
For weapon placement the map maker must return to their original idea of key points and understand where they want the battles to be fought so to speak. From there they must place key weapons making certain to balance the power weapons in these spots or around the map where they want players to explore, otherwise the balance of the key points will be lost. It is also important to keep in mind that players will need average weapons to be sprinkled around as well. Many maps make no use of vehicles and this is absolutely okay. To place vehicles on the map means to give the players that ability to have drastically increased firepower and mobility, so there needs to be some balance, either with another vehicle or with power weapons.
Beyond that, after map construction and weapons placement the final thing the map maker must do is to actually test the map. This means not only playing it as the map was meant to be played, but also pushing the boundaries to make certain no errors occurred during construction.
Often the biggest problem with in-game map editors, and usually the reason that game developers do not put them into games is the inundation of poor maps. Many players will fool around with the map maker with out following the planning steps only to create maps that are not as good as those that took many hours to create. What usually happens is that the game designers will find some way of leaving it up to the community to decided what good maps are, and work from there. Examples of this are Far Cry 2's rating system, and the map community portal of Halo 3's Forge.
Another problem that often arises is the fact that map makers are unable to fulfill their own imaginations. They design a map. but in execution the map becomes distorted with unnecessary objects. This problem does less to inundate the community with unnecessary maps as much as is prevents what could potentially be really good maps from making it to the public. The solutions to this usually revolve around having other players play on the maps and then going in and revising. Often times multiple people will coordinate to work on one map together that way no one person's idea can be distorted by their own point of view.