A compresser controls the overall volume of your guitar signal. If used too much, this will kill your tone. but it can be used for some very cool volume swell effects, play real soft and it will bring your volume up to level.
A phaser adds a set number of tone drops to your guitar signal. Sonically speaking this can either sound like a rotary speaker cabinet (real smeary tone) or, when turned up, it sounds like a ray gun.
An overdrive sounds like an amp turned all the way up.
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