Tremolo has a number of meanings for the guitarist. Tremolo bridges (like in a Fender strat.) allow you to wobble the pitch of a note. In some amplifiiers (and effects pedals) you also have a tremolo effect which varies the volume to give you a wobbly noise reminiscent of 50's and 60's guitar instrumental music (think of the guitar at the start of the Twin Peaks theme). You could achive a similar effect by playing with the volume control on your guitar.
This project shows a circuit schematic which you could use to make your own tremolo effect. This is slightly different from the oldtime 50's tremolo sound, which varies the volume in a triangular wave fashion. This circuit gives a chopper tremolo effect (sort of a square wave, on/off sound). The maker says this tremolo effect can be heard in the beginning of Plant Telex by Radiohead, or when Tom Morello of Rage Against the Machine uses his "kill switch".
