Guitar effects are electronic devices that modify the tone, pitch, or sound of an electric guitar. Effects can be housed in effects pedals, guitar amplifiers, guitar amplifier simulation software, and rackmount preamplifiers or processors. Electronic effects and signal processing form an important part of the electric guitar tone used in many genres, such as rock, pop, blues, and metal.
Guitar effects are also used with other instruments in rock, pop, blues, and metal, such as electronic keyboards and synthesizers. Electric bass players use bass effects, which are designed to work with low-frequency tones of the bass.
Distortion is an important part of an electric guitar's sound in many genres, particularly for rock, hard rock, and metal. A distortion pedal takes a normal electric guitar signal and distorts the signal's waveform by "clipping" the signal. There are several different types of distortion effects, each with distinct sonic characteristics. These include overdrive/distortion (or vacuum tube-style distortion), overdrive/crunch, fuzz, and hi-gain.
Overdrive distortion is a well known distortion. While the general purpose is to emulate classic "warm-tube" sounds, distortion pedals such as the ones in this list can be distinguished from overdrive pedals in that the intent is to provide players with instant access to the sound of a high-gain Marshall amplifier such as the JCM800 pushed past the point of tonal breakup and into the range of tonal distortion known to electric guitarists as "saturated gain." Although most distortion devices use solid-state circuitry, some "tube distortion" pedals are designed with preamplifier vacuum tubes. In some cases, tube distortion pedals use power tubes or a preamp tube used as a power tube driving a built-in "dummy load." Pedals designed specifically for bass guitar are also available.
Some distortion effects provide an "overdrive" effect. Either by using a vacuum tube, or by using simulated tube modeling techniques, the top of the wave form is compressed, thus giving a smoother distorted signal than regular distortion effects. When an overdrive effect is used at a high setting, the sound's waveform can become clipped, which imparts a gritty or "dirty" tone, which sounds like a tube amplifier "driven" to its limit. Used in conjunction with an amplifier, especially a tube amplifier, driven to the point of mild tonal breakup, short of what would be generally considered distortion or overdrive, these pedals can produce extremely thick distortion sounds much like those used by Carlos Santana or Eddie Van Halen. Today there is a huge variety of overdrive pedals like the Behringer OD400
Fuzz was originally intended to recreate the classic 1960's tone of an overdriven tube amp combined with torn speaker cones. Oldschool guitar players (like Link Wray) would use a screwdriver to poke several holes through the paperboard part of the guitar amp speaker to achieve a similar sound. Since the original designs, more extreme fuzz pedals have been designed and produced, incorporating octave-up effects, oscillation, gating, and greater amounts of distortion. Try the Behringer UZ400
Hi-Gain (descended from the more generic electric guitar amplification term high-gain) is the sound most used in Heavy Metal. High gain in normal electric guitar playing simply references a thick sound produced by heavily overdriven amplifier tubes, a distortion pedal, or some combination of both--the essential component is the typically loud, thick, harmonically rich, and sustaining quality of the tone. However, the Hi-Gain sound of modern pedals is somewhat distinct from, although descended from, this sound. The distortion often produces sounds not possible any other way. Many extreme distortions are either hi-gain or the descendents of such. The Mesa Boogie Triple Rectifier Series of amps are an example. Try the Behringer XD300
or the HM300
An Equalizer adjusts the frequency response in a number of different frequency bands. A graphic equalizer (or "graphic EQ") provides slider controls for a number of frequency region. Each of these bands has a fixed width (Q) and a fixed center-frequency, and as such, the slider changes only the level of the frequency band. The tone controls on guitars, guitar amps, and most pedals are similarly fixed-Q and fixed-frequency, but unlike a graphic EQ, rotary controls are used rather than sliders.
Most parametric EQ pedals (such as the  Boss PQ-4) provide semi-parametric EQ. That is, in addition to level control, each band provides either a center frequency or Q width control. Parametric EQs have rotating controls rather than sliders.
Placement of EQ in a distortion signal processing chain affects the basic guitar amp tone. Using a guitar's rotary tone control potentiometer is a form of pre-distortion EQ. Placing an EQ pedal before a distortion pedal or before a guitar amp's built-in preamp distortion provides preliminary control of the preamp distortion voicing.
For more complete control of preamp distortion voicing, an additional EQ pedal can be placed after a distortion pedal; or, equivalently, the guitar amp's tone controls, after the built-in preamp distortion, can be used. An EQ pedal in the amp's effects loop, or the amp's tone controls placed after preamp distortion, constitutes post-distortion EQ, which finishes shaping the preamp distortion and sets up the power-tube distortion voicing.
As an example of pre-distortion EQ, Eddie Van Halen places a 6-band MXR EQ pedal before the Marshall amplifier head (pre-distortion EQ). Slash places a Boss GE-7, a 7-band EQ pedal, before his Marshall amp. This technique is similar to placing a Wah pedal before the amp's preamp distortion and leaving the Wah pedal positioned part-way down, sometimes mentioned as "fixed wah," (pre-distortion EQ), along with adjusting the amp's tone controls (post-distortion EQ).
If a dummy load guitar-amp configuration is used, an additional EQ position becomes available, between the dummy load and the final amplifier that drives the guitar speaker. Van Halen used an additional EQ in this position. This configuration is commonly used with rackmount systems.
Finally, an EQ pedal such as a 10-band graphic EQ pedal can be placed in the Insert jack of a mixer to replace the mixer channel's EQ controls, providing graphical control over the miked guitar speaker signal.
Equalization-related effects pedals include Wah, Auto-Wah, and Phase Shifter. Most EQ pedals also have an overall Level control distinct from the frequency-specific controls, thus enabling an EQ pedal to act as a configurable level-boost pedal. Some EQ pedals include the Behringer EQ700
A Phase Shifter creates a complex frequency response containing many regularly-spaced "notches" in an incoming signal by combining it with a copy of itself out of phase, and shifting the phase relationship cyclically. The phasing effect creates a "whooshing" sound that is reminiscent of the sound of a flying jet. This effect dominates the sound in the song Star Guitar by Chemical Brothers. The song was not played with any guitars but you can hear the phasing effect. The instrument being phased was actually a synthesizer. Some electronic "rotating speaker simulators" are actually phase shifters. Phase shifters were popular in the 1970s, particularly used with electric piano and funk bass guitar. The number of stages in a phase shifter is the number of moving dips in the frequency response curve. From a sonic perspective, this effect is equalization-oriented. However, it may be derived through moderate time-based processing. Some phaser pedals include the Behringer PH9Wah pedal
A Wah-wah pedal is a foot-operated pedal, technically a kind of band-pass filter, which allows only a small portion of the incoming signal's frequencies to pass. Rocking the pedal back and forth alternately allows lower and higher frequencies to pass through, the effect being similar to a person saying "wah". The wah pedal, used with guitar, is most associated with 1960s psychedelic rock and 1970s funk. During this period wah-wah pedals often incorporated a fuzzbox to process the sound before the wah-wah circuit, the combination producing a dramatic effect known as fuzz-wah. Try the Behringer HB01
Tremolo is a regular and repetitive variation in gain for the duration of a single note, which works like an auto-volume knob. This is a volume-related effects pedal. This effect is based on one of the earliest effects that were built into guitar amplifiers. Examples include the Behringer TP300
A compressor acts as an automatic volume control, progressively decreasing the output level as the incoming signal gets louder, and vice versa. It preserves the note's attack rather than silencing it as with an Envelope Volume pedal. This adjustment of the volume for the attack and tail of a note evens out the overall volume of an instrument. Compressors can also change the behaviour of other effects, especially distortion. When applied toward the guitar, it can provide a uniformed sustained note; when applied to instruments with a normally short attack, such as drums or harpsichord, compression can drastically change the resulting sound. Try the Behringer CS100
Reverb is the persistence of sound in a particular space after the original sound is removed. When sound is produced in a space, a large number of echoes build up and then slowly decay as the sound is absorbed by the walls and air, creating reverberation, or reverb. A plate reverb system uses an electromechanical transducer, similar to the driver in a loudspeaker, to create vibration in a plate of sheet metal. A pickup captures the vibrations as they bounce across the plate, and the result is output as an audio signal. A spring reverb system uses a transducer at one end of a spring and a pickup at the other, similar to those used in plate reverbs, to create and capture vibrations within a metal spring. Guitar amplifiers frequently incorporate spring reverbs due to their compact construction. Spring reverberators were once widely used in semi-professional recording due to their modest cost and small size. Due to quality problems and improved digital reverb units, spring reverberators are declining rapidly in use. Digital reverb units use various signal processing algorithms in order to create the reverb effect. Since reverberation is essentially caused by a very large number of echoes, simple DSPs use multiple feedback delay circuits to create a large, decaying series of echoes that die out over time. A good example would be the Behringer DR100Delay/Echo
A Delay or Echo pedal creates a copy of an incoming sound and slightly time-delays it, creating either a "slap" (single repetition) or an echo (multiple repetitions) effect. Delay pedals may use either analog or digital technology. Analog delays often are less flexible and not as "perfect" sounding as digital delays, but some guitarists argue that analog effects produce "warmer" tones. Early delay devices actually used magnetic tape to produce the time delay effect. U2's guitarist, The Edge, is known for his extensive use of delay effects. Some common Delay pedals are the Behringer EM600
Chorus splits your guitar's signal in two, modulating the second signal's pitch and mixing back in with the "dry" original signal. The effect sounds like several guitarists playing the same thing at the same time, resulting in a wide swelling sound. Try the Behringer UC200
A multi-FX pedal is a single effects device that can perform several guitar effects simultaneously. Such devices generally use digital processing to simulate many of the above-mentioned effects without the need to carry several single-purpose units. In addition to the classic effects, most have amplifier/speaker simulations not found in analog units. This allows a guitarist to play directly into a recording device while simulating an amplifier and speaker of his choice.
A typical digital multi-effects pedal is programmed, with several memory locations available to save custom user settings. Many lack the front-panel knobs of analog devices, using buttons instead to program various effect parameters. Multi-effects devices continue to evolve, some gaining MIDI or USB interfaces to aid in programming. Try the Behringer FX600