I'm sure this is covered in other threads, so look around, but in my opinion, here's the basics:

Trance has a lot of electronic instruments, and synths, and basslines, and often vocals.

Techno has a much more pronounced beat, for the most part, and the beat isn't as regular as it is in Trance... There's also more electronic sounds...

House is much more focused on the beat, and has much less music. The beats and rhythms are a lot more complicated, and a lot more repetative.


Side note, there's very rarely distortion in trance music, except in some goa or psy trance... Techno has quite a bit, and house varies..

I could get flamed for my descriptions, but it's been fun.. heheh.

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Electronica Primer
Although not everyone will agree with everything here, it is still agreat site for newbies to sort through the genre morass and give some lessons in the roots of the music.

im gonna generalize so you get a quick feel:
trance TENDS to have those melodic synths and huge buildups (like atb or ayla, two of the most well-known)

house TENDS to have that simple beat, or a disco-ish feel. or tribal (also lots more vocal samples). think club.

techno is a whole bunch of beats, lots of percussion and noises to create the music, NOT melody-based. so you usually cant hum a techno song.

that should give you a quick primer there..

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what trainspotters are.