Volume effect does exactly as the name applies, it changes the volume in a specified way.

1. Volume pedal is a foot pedal that adjusts the output signal volume by raising or lowering a foot pedal
2. Envelope filter is like an autowah except it is manipulating volume instead of tone
3. Auto swell pedal can create some really dramatic volume swell effects, listen to Fleetwood Mac song "Dreams" guitar intro.
4. Tremolo is a cyclic pattern of volume set by a timing parameter