Reverb, short for reverberation, is an effect that alters the guitar's sound to recreate the sound of playing in a room or other environment.
Reverb is a natural occurrence for all sounds and noticeably occurs when sounds are acoustically different in different rooms due to the way the sound carries or is reflected. These environments factor into how long the reverberation lasts or fades out. Reverb is one of the most common effects used in recordings and as effects for different instruments.
Spring Reverb
Built-in spring reverb units were first used in Hammond Organs in 1939, and was adopted for guitar amplifiers using spring reverb tanks inside the guitar amp. The different sizes of reverb tanks allows them to be placed inside the cabinet of combo amps and in guitar amp heads.
Spring reverb units are made with a metal enclosure containing metal springs stretched to each side of the enclosure, connected to a transducer on one end and a pickup (similar to those used in plate reverb) on the other end. The sound of the guitar's signal is sent through and causes the springs to vibrate creating a reverb effect.
Stand-Alone Reverb Units
Stand-alone reverb units usually have longer springs, allowing for more reverb to be produced through them, and often look similar to a guitar amp head. These reverb units became popular with surf guitar.
Analog Reverb Effects
Analog reverb effects are produced by several different manufacturers. Since these pedals use analog circuitry, they aren't as sonically flexible as Digital Reverb effects. Both come as pedals or stomp boxes, and in rack mountable units.
Digital Reverb
Digital reverb units process the guitar signal to create spring reverb type effects as well as more extreme reverberation effects for more ambient sounds, but sometimes lack the warmer tone analog effects produce. Digital reverb effects can be found in studio software, rack units and stompboxes and allow the user to simulate different rooms or environments to process the sound.