The Room Editor (found as a Windows executable file called winroomedit.exe in the installation path of the level editor) is the heart of the Level Editor. It (or it unofficial successor, the Next Generation Level Editor) is used to build the map of any the many Custom Levels available on the internet today.
The Room Editor - as well as all the other tools provided - are somewhat user unfriendly, as they were originally never intended to be released to the public. Older versions of the software have been used to create the official Classic games (TR 1 through 5) by Core Design.
A modernized version, the Next Generation Room Editor, is part of the unofficial Next Generation Level Editor.
What the Room Editor is NOT
The Room Editor does not provide the ability to create new objects, re-texture existing objects, or mix objects from various WADs. These operations can only be performed with one of the countless third party Levelbuilding Tools.