hey there lodger,
trance is a form of dance music that came from germany in the early 90's. it is a form of music that derived from house and techno. early trance was very genre specific. where d&b, and breaks relied heavily on their rhythms, early trance relied on its uplifting melodies; and its simple 4-on-the-floor beats were used to create a dance/dj friendly repetetive beat that... well, put people in a trance.
as trance started evolving, new sub-genres of the style started to come about such as: tribal, dark, dream, goa/psy, progressive, hard, trance/breaks, etc. however, these sub-genres of trance seem to have a few things in common: a simple house/techno rhythm, a repetetive/grooving bassline, the presence of acid derived 303's, progression (where simple intros evolve until layers upon layers of melodies and or rhythms are added on top of eachother) and a ton of ethereal effects (echos, reverbs and the like).
trance today is breaking (or at least bending all of the rules of early trance), because the nature of this genre is always moving, always evolving. however, there seems to be three dominant schools of REAL trance categories that most tracks fall in to, they are the: dark/experimental school led by sasha & digweed, the uplifting/hard dutch school led by tiesto and corsten, the (darker than dutch, but lighter than sasha, groundbreaker) school of club led by PVD and oakenfold, and the goa/psy school that was championed by astral projection.
in my opinion, there are no rules in electronica, but there are rules in trance. there's nothing wrong with breaking these rules, but then you stray further and further away from what trance really is. just like if you stray further away from the methods of jazz, the product is still music, but no longer true jazz.
hope this helped, and sorry this post was so long...
peace,
hypnotik
http://www.mp3.com/hypnotik