I love the smell of UnrealEd crashing in the morning. – tarquin
Unreal Editor overview
From Unreal Wiki, The Unreal Engine Documentation Site
The Unreal Editor, UnrealEd or UEd for short, is the game content editor for Unreal, which is where the name comes from, and most other Unreal Engine games. Each Unreal Engine game has its own version of the editor, which is usually named after the game it comes with, for example XIIIEd for the game XIII or Unreal Tournament 3 Editor for UT3.
Due to its deep integration into the engine, the editor applications of different games (or sometimes even different versions of the same game) are usually not compatible to each other. That means if a certain game does not come with the editor, it is very unlikely that another game's editor could be used instead. Also, the various developers may have made customizations to their specific versions of the editor.
One common "feature" of all UnrealEd incarnations is their lack of stability. While the editor has become more stable over time and has an auto-save feature for maps, you should always work under the motto "save early, save often!" As a rule of thumb you should save your maps and custom resource packages right before and after every big operation.
 Major versions
 Unreal Engine 1
Early Unreal Engine 1 games come with UnrealEd 1, later games with UnrealEd 2. Some games, including UT, originally included UnrealEd 1, but switched to UnrealEd 2 in a later patch. UnrealEd 1 and 2 differ greatly in their overall look and handling.
 Unreal Engine 2
- Main article: UnrealEd 3
Most Unreal Engine 2 games, including UT2003 and UT2004, include UnrealEd 3. It is similar to UnrealEd 2 in its general look, but was improved quite a bit and provides tools for working with the new engine features like terrain and static meshes.
 Unreal Engine 3
- Main article: Unreal Engine 3 Editor
After the confusing version numbers in earlier generations (e.g. UnrealEd 2 ≠ Unreal Engine 2) the number was dropped entirely for Unreal Engine 3 and the editor is just named after its corresponding game, for example Unreal Tournament 3 Editor.
 General features
The main feature of all UnrealEd versions is, of course, the map editor. Other features include various browsers that allow importing resources like textures or sounds into packages and editors that allow compiling UnrealScript classes, creating complex materials, particle emitters and so on.
The various editor features are exposed in very different ways in the different editor versions. For example, mesh import was only possible through console commands or UnrealScript code in UnrealEd 1 and 2. The Unreal Engine 3 editors no longer provide means to edit and compile UnrealScript classes but have a combined generic browser and some very elaborate editors for materials, particle effects and many other features.