User maps are fan-made levels created for a computer game. This article talks primarily about user maps made for Duke Nukem 3D, since it is by far the largest fan community around.
User maps are created the same way as the maps included in the game, i.e. by using the level editing program Build.
As technology develops, it becomes possible to expand on the limits of maps, such as increasing the maximum number of walls allowed in a map, and the number of sprites allowed on the screen at once. This increasing of the limits is often made possible through the use of a source port. More recent user maps will often take advantage of the larger limits to create bigger and more elaborate maps, but they will then not be able to be played without the source port to support the larger limits.
Also related to growing technology is the issue of compatibility. Old user maps are often not compatible or run incorrectly under newer source ports (which are often necessary to be able to play the game at all). This is due to the source port being changed to no longer support features the old user map used, or to make those features perform differently.
However, if the user remains up to date, improving technology is only a good thing. That user can create bigger and better maps, and has access to little tricks the source ports have that can produce effects that weren't possible with the original game engine. New tricks can also be discovered by fellow mappers in the community, and being part of that community allows the user to learn these new tricks.
As a result of the improved capabilities of technology and the source ports, it is not uncommon to see recently-made user maps with far more detail than the original levels.