Trance music is a subgenre of electronic dance music that developed in the 1990s.
Perhaps the most ambiguous genre in the realm of electronic dance music (EDM),
trance could be described as a melodic, more-or-less freeform style of music derived
from a combination of techno and house. Regardless of its precise origins, to
many club-goers, party-throwers, and EDM adherents, trance is held as a
significant development within the greater sphere of (post-)modern dance music.
While there is no strict definition for "trance," songs of this genre are usually characterized as being accessible and having "anthemic" qualities. Using that as a starting point, a basic trance track could then be described as being comprised of a particular melodic and/or vocal hook which is given presence over an uncomplicated bassline, a simple drum pattern (which often includes snare and/or kick drum rolls to mark "big moments"), and perhaps one or two other semi-quantified aural elements to provide texture and enhance the rhythm. Trance also usually features more complicated chord progressions and melodies than were found in the music at the time, including 4 chord progressions symptomatic of 80's new wave music. However, not all trance fits that profile, and often times a song's classification as "trance" has just as much to do with who is playing it as what it sounds like.
At present (and as alluded to earlier), trance is as much about who plays the music as it is about what it sounds like. Many artists described as producing a very powerful trance sound (e.g., Underworld's "Cowgirl" from 1994 remains a floor-filler) have most recently released tracks more suggestive of techno (Underworld's "Moaner" from 1998); DJs like John Digweed, known for spinning scintillating trance anthems in 1996, turn to a darker, housier sound in 2000. All the while, new artists and DJs enter the fold, either taking over the vacancies left in the anthemic, "progressive" arena (e.g., DJ TiŽsto and ATB), or else introducing new forms, modes, and themes (e.g., Sander Kleinenberg and Steve Lawler).
Thanks to http://en.wikipedia.org/