DED (Doomsday Engine Definition) is the name of the textual definition language of the Doomsday Engine. One syntax to define everything from Particle Generators to Things, in an easy to edit plain text format.
Files in this format are conventionally given the .DED file name extension.
This article provides a high level overview of the definition language, including the most important elements in a .DED file - the definition types themselves.
Typically a .DED file consists of a collection of definitions, either loosely related, or, grouped together according to some organizational structure (normally decided upon by the mod author(s)).
The Doomsday definition file parser maintains a dictionary of all definitions encountered thus far (and which was the last, for each type). This database allows for definitions to be constructed piecewise or in a non-linear fashion.
If you are new to Doomsday modding you'll want to familiarize yourself with DED Syntax before going further. (It won't take long as its fairly standard/straightforward).
See also: Altering and copying definitions
Definition files may be specified to the engine for loading in a variety of ways:
See also: Paths on the command line
These articles offer complete walk throughs on how to achieve something very specific within the realms of editing for Doomsday with step-by-step guidance. Plus background information on "why" a certain method has been used.