Effects are qualities added to an amplified sound by electrical modification of the signal. Some commonly applied effects are
Wah - moves a peak in frequency response up & down the frequency spectrum
Phasers - automatically moves notches in the frequency response up and down the frequency spectrum (notches spaced evenly across the frequency spectrum).
Compression - makes loud passages quieter, and quiet passages louder.
Overdrive & Distortion - produces the sound of an amplifier overdriven to the point where it 'clips' signal
Equalization - provides more tone control than bass, middle and treble controls. Graphic and parametric equalizers work differently: Graphic equalizers use sliders to control the level at a fixed range or band of frequencies. The position of the sliders provides a graphic representation of the relative contributions of each band of frequencies. Parametric equalizers generally provide a bass and treble control that allow broad tonal shaping that is further refined by one or more middle controls, each defining three things: 1) Frequency: a range of frequencies boosted or diminished, 2) Resonance: the width of the range of frequencies affected, and 3) Level: the amount of boost or cut applied.
Harmonizers - electronically add one or more notes to the original note or notes. Early harmonisers added a distorted signal one or more octaves below a singal note. More recently developed harmonisers digitally store the tone and timbre of the original note or notes and produce both single note and chord harmonies at user-defined intervals from the original tone.
Vibrato - varies the pitch smoothly between slightly flat and sharp, emulating the fingerboard technique of vibrato.
Flanging - mixes a varying delayed signal with the original to produce a series of notches in the frequency response (notches are harmonically spaced).
Chorus - mixes a varying delayed signal with the original to produce a large number of harmonically related notches in the frequency response (early chorus was indistinguishable from flange, current chorus uses a longer delay than flange)
Volume Pedals - controls the volume.
Tremolo - rapidly modulates volume up and down.
Delay - replays what a note or notes one or more times after a set period of time.
Reverb - consists of a very large number of repeats, with varying levels and tones over time that are designed to emulate the complex reflection of sound from the hard surfaces in a room. Modern reverbs offer a choice of several digital algorythms that simulate small and large rooms and halls, studio effects, plate, chamber and reverse * reverbs, or spring reverbs (the earliest reverb was actually a spring in the amplifier...a technique still commonly used).