Modulation is a category of effects, a collective of effect types that manipulate the sound in a particular way so phaser, flanger, chorus, tremolo etc would all be considered modulating effects.
CHORUS effects slowly modulate delay time with a low frequency oscillator (LFO). As the LFO cycles the delay time up and down, the delayed volume creates a chorus !
TREMOLO creates dynamic pulsing that gives modulation control over volume Intensity and adds a groovy tone wobble.
PHASE In the world of effects, the phaser is the most popular it creates the swirly whirly rotating sweeping sounds.
FLANGER Prepare to take off, the flanger takes your guitar tone by jet propulsion sweeping levels low and high. WOOSH.
AUTO WAH The Auto Wah is expressed by how hard or soft you play. The pedal sweeps the peak response of a filter up and down in frequency, creating the Wah sound