Before we begin, here's a useful tidbit of info that will help... tremolo is the effect of varying volume, whereas vibrato is the effect of varying pitch, two effects that are often mistaken for one another or considered the same thing.
Tremolo pedal’s employ modulation to vary the volume of your signal at a rhythmic, steady rate, the speed of which you can control. The tremolo effect was an early favourite of guitarists back in the ‘50s and ‘60s, with manufacturers such as Fender including it in their amplifiers as standard. The effect was paramount to the rock’n’roll, surf and rockabilly sounds of the era.
Vibrato Pedals work similarly but have a little of a chorusy flavour thrown into the mix. The difference is, a chorus pedal splits the signal, applying the pitch modulation to one of them before offsetting and mixing the signals back together. A vibrato pedal simply applies the same effect to the whole signal. The resulting effect is a warbling, natural sound that can add a huge amount of warmth and body to your sound.