UDMF is a new (text based) map format that allows greater editing flexibility and does away with a number of the old map format restrictions.
http://zdoom.org/wiki/Universal_Doom_Map_Format
http://doom.wikia.com/wiki/UDMF
http://doomwiki.org/wiki/UDMF
(I can never remember which Doom wiki is the better one)
[edit] beaten to it.
