Room Editor
From WikiRaider
The Room Editor (Windows: winroomedit.exe) is the heart of the Level Editor. With it you build the map for your level.
The Room Editor is used to create environments, texture the scenery, place Objects, set Triggers and more.
What the Room Editor is NOT
The Room Editor cannot create new objects, retexture them, or mix objects from various WADs. These operations can be performed with one of the countless Levelbuilding Tools.