USB transfers data over a differential pair, D+ & D-.
Differential pairs allow more noise or attenuation while still working.
When D+ is more positive than D- the signal is differential 1.
When D- is more positive than D+ the signal is differential 0.
A 'balanced line' is faster & less error prone than a rigid protocol like RS232.
Too add to what Mosfet said (and maybe clear it up a bit) when the first data line is "higher" than the second, its a 1, and if the second line is "higher" than the first its a 0, you can almost think of this as just a single data line. There is no dedicated clock line though, its mixed in with the data line and transferred at the same time. Unfortunately I've forgotten the specifics of it though.
If I remember correctly, USB sends out all the data to every device on the chain, and the devices all have an ID number associated with them. They look for the ID number in the data/clock stream and if it is theirs, they take the data.