Modulation is the effect that alters the signal and boosts/strengthens some characteristics of a signal. Some of these characteristics can be phase, frequency and pitch. The different kinds of modulation effects are: chorus, phaser, flanger, ring modulator, Vibrato and Tremolo. The first ever pedal that was commercially available was the DeArmond Trem-Trol which in fact is a tremolo pedal.
Starting of with the chorus effect, the main purpose of this effect is for it to mimic/simulate the choirs that can be found in stringed instruments that are used in orchestras such as Violins. This is done by adding slight variations in pitch and timbre. The effect it self also splits the signal and adds some delay to one of the signals to simlulate a vibrato. An example of a well known chorus effect pedal would be the Boss CE-1.
Now moving onto phasers, this effect was first made to imitate the effects of a rotary speaker. This was done in order to achieve phase shifting and in order for this to be achieved the signal has to be split into two and have one of the signals modulated and leaves the other as it is. This then creates a rippling effect which amplifies some characteristics of the signal. A example of a well known phaser pedal would be the Uni Vibe.
In comparison to the previous two effects, flanger's also split the signal into two and modulate one of them, leaving one of the signals as an original. The modulated signal is then delayed and has the Doppler effect added to it in order to modulate the frequencies. This is done to emulate the sound of a "Jet plane" passing by. Some examples of flanger effect pedals are: MXR Flanger and Electro Harmonix Electronic Mistress.
On the other hand ring modulator's do not split the signal but they mix the original audio signal with a carrier wave which is produced by the device's oscillator. This then generates a resonnant and metallic sound due to the fact that the original sound wave is suppressed. Here are a few examples of famous ring modulators: Mooger-Fooger MF-102 and Digitech/DOD Gonkulator Ringmod.
Moving onto Vibrato effects, these produce small and fast alterations in pitch in order to simulate slight semitones variations that are naturally naturally generated by opera singers. This then mimics the sound of bending a note on a guitar giving you a trilling effect. A example of a well known vibrato pedal is the Dunlop MXR Uni Vibe.
Lastly lets talk about the tremolo effect pedal. The tremolo effect creates a pulsating effect at different speeds which can be compared to the sound of a helicopter. This is done by adjusting the amplitude of the original signal. A well known Tremolo unit is the DeArmond Trem-Trol and the Fender Tremolux.