Some of the earliest effects available to guitarists were tremolo and vibrato. These effects were usually built into the amplifier. Between the vibrato, tremolo, and reverb there was little else available to the electric guitarist.
Leo Fender's company while prominent in the development of the electric guitar led to some great confusion with the mis-labeling of their amps tremolo channel as vibrato.
Seasoned guitarists have figured this out but for anyone who hasn't quite nailed down the difference here is a quick video to demonstrate to the difference between tremolo and vibrato.
