Tremelo is often used incorrectly to describe Strat bridges which are actually Vibrato bridges.  Vibrato the rapid raising and lowering of the pitch, and is thus fundamentally different from tremolo which is a rapid variation in only the loudness of the sound.  Guitar bridges should be correctly described as Vibrato bridges, not tremelo which is the amplifier effect Bo Diddley used to create his Diddley beat sound back in the 50's Bo Diddley Mona http://www.youtube.com/watch?v=luMQUzJzZLQ  Vibrato with BB King  http://www.youtube.com/watch?v=vg7ugDja1fA