Compared with reverb, the delay effect is less complex and easy to understand. The delay effect "stores" the input sound, and outputs it after a preset time interval.
All delay units allow you to specify the delay time (the time space between the input note and the output), how many times the note is repeated, and the intensity with which is repeated.
Most delay unit have additional controls. The feed-back control determines to which extent the sound intensity should be diminished each time the sound is repeated. Every time the delay signal repeats, it becomes quiter.
Analog versus digital delay
Many of the first delay processors were based on analog recording systems: the sound was recorded, usually on magnetic tape, and played. The effect could be manipulated by controlling tape speed variations. Though still in demand by retro-sound seeking guitar players, most delays are nowadays produced by digital devices.
Digital delay implies the sampling of the sound by analog to digital converters, and after manipulation, (usually) re transforming it into analog sound. Digital delay is offered by many vendors, both as standalone units such as the famous Boss pedals, and built in into a multi effects unit. Delay effects are obviously also available in the computer based recording software environment.
Delay on violin
A delay is a very interesting effect for violin. It helps you to quicly fill up a space. using a short delay time you can create some kind of "doubling" effect. One violin player can build a sound like two players with two violins playing simultaneously. But be careful: as any effect, you can quickly overload your sound with delays. This is very tempting, but makes music more heavy to listen to.