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 techno. Or house. Maybe both. Regardless, 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. 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.
A brief history of trance music
As a genre, trance is said to have begun as an off-shoot of techno in German clubs during the early 1990s. The name derived in 1991 from a project of Dag Lerner (DJ Dag) and Rolf Ellmer (Jam El Mar) called Dance2Trance. Their song "We Came In Peace" also set the original definition of trance music, a drawn out and monotonous pattern with a short but repeating voicesample. The sound was meant to work hypnotic to the listeners. Arguably a fusion of techno and house, early trance shared much with techno in terms of the tempo and rythmic structures but also added more melodic overtones which were appropriated from the style of house popular in Europe's club scene at that time. (Interestingly enough, that style of house was referred to as "club" or "Euro.") However, the melodies in trance differed from Euro/club in that although they tended to be emotional and uplifting, they did not "bounce around" in the same way that house did. This early trance tended to be characterized by the anthemic qualities described above, and typically involved a break-down portion of the song in which the beat was dropped for a few bars to focus on the melody before bringing the beat back with a renewed intensity. The sounds used in trance tended to be produced by analog synthesizers (or recently, digital simulations of analog synthesizers, often called virtual analog synthesizers), with lush "strings" providing the basis for the melodies and pads, while similar analog equipment was used to produce basic bass notes and the regimented "four-on-the-floor" drum loops. This style became instantly popular in Europe and spread very quickly. Before long, trance was spawning sub-genres such as dream trance, acid trance, hard trance, and goa. (NOTE: Goa and psy-trance are arguably older, with their characteristic sounds purportedly emerging in Israel as far back as 1991.)
By the mid-1990s, trance had emerged commercially as one of the dominant genres of EDM. Immensely popular, trance found itself filling a niche as edgier than house, more soothing than drum-n-bass, and more accessible than techno. By this time, trance had become synonymous with progressive house and both genres essentially subsumed each other under the commercial banner of "progressive." Artists like Brian Transeau (BT), Paul Van Dyk, Ferry Corsten (Art of Trance), and Underworld came to the forefront as premier producers and remixers, bringing with them the emotional, "epic" feel of the style. Meanwhile, DJs like Paul Oakenfold, Sasha, and John Digweed were championing the sound in the clubs and through the sale of pre-recorded mixes. By the end of the 1990s, trance remained commercially huge but had fractured into an extremely diverse genre. Some of the artists that had helped create the trance sound in the early and mid-1990s were, by the end of the decade, branching out with more experimental work (artists of particular note here are BT and Underworld, the latter of which was defunct by 1998). Perhaps as a consequence, similar things were happening with the DJs as well; for example, Sasha and Digweed, who together had helped bring the progressive sound to the forefront, all but abandoned it by 2000, instead spinning a darker mix of the rising "deep trance" style (as marked by the duo's 2000 release, "Communicate").
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., Tïesto and ATB), or else introducing new forms, modes, and themes (e.g., Sander Kleinenberg and Steve Lawler).
For more concrete examples, check out any number of purported trance compilations; perhaps the most highly recommendable source would be the Global Underground series (http://www.globalunderground.co.uk), including its "Nubreed" sub-series, because it captures the diversity of the genre as expressed through many of its brightest DJ talents. Also recommended as source material would be the Tranceport/Perfecto Presents... series, any of Sasha & Digweed's Northern Exposure mixes, and any of the mixes in the Renaissance series. Labels to reference would include 3Beat, Bedrock, Devolution, Fluid, Fragrant, Hooj Choons (http://www.hoojchoons.co.uk/), Hook, Perfecto, Positiva, and Yoshi Toshi(http://www.yoshitoshi.com).