Overview of EDGE Editing
MappingEDGE is a DOOM engine, and hence you create new levels for it by using normal DOOM editing tools. There are quite a few of them actually, but they fall into two main categories: Level editors and Wad editors.
EDGE is also a Boom-Compatible port, hence you can use Boom features in your maps as well. We are not quite 100% compatible though, see the 'Compliance' page for more information and the list of differences.
Using EDGE-specific features in a map requires that you use an EDGE Line type or Sector type, plus there are a couple new Thing types as well. This editing guide contains some handy lists of all the available types that EDGE supports.
Extrafloors (also called "3D Floors") are one of EDGE's coolest features, removing the limitation of DOOM mapping which doesn't allow rooms over rooms. EDGE can also make Liquids, with translucent surfaces and color-tinted areas, with various properties (like damaging or swimmable).
New Weapons and Monsters
Currently the DDF documentation is being reworked and is not available yet. The "unofficial" version can be found HERE.
DeHackEd patches are a old method for modifying the behaviour of weapons and monsters, and for changing text strings. People sometimes still use it because it is supported by most DOOM ports. EDGE has excellent support for DeHackEd patches. See the 'Compliance' page for more details.
ScriptingEDGE has a basic scripting language called "Radius Trigger Scripting", or just "RTS" for short. The basic idea is to execute some commands whenever the player enters a certain area (radius) on the map. Other types of triggers are also available, such as executing the commands when all the monsters of a given type are dead.
RTS lacks some useful things that more powerful scripting languages enjoy, such as variables and calculations. Nevertheless people have created some amazing things with it.
For more information on RTS, click on the 'RTS Online' link in the menu.