The major difference to mainstream dnb (a gap which is fast closing as more dnb artists break into the traditionally pop-dominated charts) is the ‘mood’. Ascending and descending semitones induce the sense of urgency or melancholy, building step by step to the typical dnb ‘drop’. Sampling comes in handy to create the ethereal atmospherics - if you can imagine the track in a sci-fi movie, you’re reaching darkstep territory.
Looking around the web, a great definition can be found on last.fm, and darkstep.org can deliver the goods as only a fan-site can, complete with upcoming darkstep releases for 2011.
Oh, and if you hear anyone chuck in techstep or neurofunk look no further than darkstep for your definition; if you become embroiled in a neurofunk vs darkstep row - agree to disagree, across pages and pages of darkstep/neurofunk debates no firm conclusion as to which is the ‘true‘ genre has been reached.
A few notable darkstep artists to get you in the mood:
Black Sun Empire
Calyx and Teebee
Future genre deconstructions include tropical, post-dubstep, juke and any more nominations by the readers - email me or leave a comment to question the genre or sub-genre you're interested in!