The diff is that trance has a kind of musicality that can't even be matched by many other genres of music--IMHO it rivals classical music in that it is as complex as it is simple.
Techno is headache-inducing repetitive music that really isn't very popular anymore. Download artists like KlubKraft of Polarium for examples. Techno has little variance, and is basically composed of a percussion line that doesn't vary throughout the entire song (usually bassdrum-hihat-bassdrum-hihat, the unh-tiss-unh-tiss-unh-tiss-unh-tiss that most people associate with electronica in general). It's got a bassline that usually isn't composed of more than 2 notes, and is played very loudly, and a very electronic melody line, often spanning less than an octave. Techno can generally be made in a 6 track mixing machine, because of the lack of depth. It is sometimes overlayed with synthesized lyrics or stupid phrases.
If you listen to trance after reading that description, you will definitely hear the difference. Trance music is rarely mixed with fewer than 24 tracks. It has varying beat structures, layered melodies often using plucked string instruments or piano/piano-clones. The bass line generally follows the melody wherever it goes, staying within an octave for the most part. And importantly, trance music always has a harmony, usually a synthesized warm string pad (a synth violin, viola, cello, or perhaps a combination), that complements the melody. It is repetitive perhaps only in its lyrics, because when listening to a trance song, you can generally hear peaks and valleys in the strength of the music which add variety to the song. Instruments are often pulled out of the song one by one until only one remains and then are fed back in as slowly, for a roller coaster kind of effect. The lyrics are usually tracked to the melody and are often sung by an airy, angelic female voice. If you really listen to a trance song, you can pick up the emotion that the artist is trying to convey. Good examples throughout the spectrum of trance include 'It's My Turn' by Angelic, 'Don't Give Up' by Chicane, 'Carte Blanche' by Veracocha, 'Greece 2000' by Three Drives, most PVD songs (I love 'The Riddle', even though they played it on MTV), Matt Darey remixes, Tillmann Uhrmacher...there's a lot.
Cliffs: Trance > Techno
, i'm not knockin' it... i just seriously don't know what the difference is. isn't it all just "uns uns uns uns uns uns un un uns uns uns....."?