This describes the necessary tools and steps to have a proper environment to create maps for Urban Terror. These steps are written with version 4.1 in mind. I'm mostly using GtkRadiant on Windows, but I'm trying to describe these steps as OS independent as possible and expect the reader to be smart enough to know how their system works.
GtkRadiant is one part in a long existing and still evolving history. It stems from the original Quake3 editor, QERadiant, and spawned several forks, e.g. NetRadiant and DarkRadiant.
Install GtkRadiant as described on the page.
GtkRadiant does not know any specifics of the various Quake3-engine based games and usually games provide a so called Gamepack. There's none for Urban Terror, but it's not necessary. Through the basic GtkRadiant configuration and a few files from the Urban Terror distribution itself, everything can be set up.
Go to the Project Settings (this dialog may actually show up the first, or even every, time you start the editor) with these values:
Further, set the proper path to the engine in the global preferences Settings / Paths / Path Settings and set it to directory of your Urban Terror installation.
Entities is a concept used within the games but also within the editor. There are game specific and editor specific entities. In the end it boils down to one or more configiration files describing these entities. Urban Terrors distribution ships one in the zpak000_assets.pk3 archive in scripts/urbanterror.def. Extract this file and put it into the folder q3.game/baseq3 from the GtkRadiant installation. Upon start up of GtkRadiant all the *.def files are parsed and provided in the editor.
As already told, there are also editor specific entities. They may be purely for the benefit of the mapper, providing him additional assets to improve his working. One such addition is the file MeasureUtil4.0.def (originally from keres.satgnu.net, thanks to keres) compilation. It provides entities with bounding boxes for varios states and actions of the player.
Others may provide alternatives to existing *.def files, so watch out for them.
Shaders can be anything from a simple texture to a real complex construct. However, they also play a significant role for the tools which compile the map. They provide valuable information where players can go, which surfaces are blocking, provide hints to optimize the compiled map, etc. They're usually called the common shaders, e.g. common/caulk, common/clip.