What is reverb?
Reverb is naturally created when a sound is reflected and absorbed by its environment, such as a room. The way the reverb sounds depends on items in its way, such as people, furniture and even air. You often talk about small or big reverbs and how to achieve them, a good ground rule is the larger the room is and the lesser it is filled with items, the bigger the reverb will get. Digital reverb is created by signal processing and can be found in music making programs, rack units or in the shape of a pedal.
Any good examples?
Every song needs reverb (at least some) or else it won't come alive, so hearing it is a part of your every day listening and you may not even think about it. But if you listen to song by Glasvegas, a band that uses a ton of reverb on their songs to make them sound really wet, and compare it to a song by ZZ Top, a band that is very reverb shy and have really dry productions, you'll hear the difference. There are a couple of different reverbs that you can use. Natural reverb is the one that is produced in a room, in studios often built round certains specifications in certain materials and using panels, bass traps and other tools to create a neat reverb. Sometimes this reverb is created inside an echo chamber and can then be descirbed as chamber reverb. Plate reverb is created by a device which uses a tranducer to change electrical sound waves to mechanical vibrations, which in turn vibrates a large plate of metal. These vibrations will then reflect back and forth before changing the vibration back into electrical sound waves, creating reverb. Spring reverb uses the same technique as the plate reverb but with the differ that instead of a metal plate this device uses springs. A magnet is moved inside the springs and this is what creates the reverb. A digital reverb unit can mimmick these different reverbs (and often more options, like reverse reverb), but the downside is that these units just calculates equations to emulate how a reverb probably would sound and doesn't really know exactly how is should sound. Still, they're very much simpler units to drag around than an actual echo chamber and modern digital pedals are so close to what they emulate it's hard to hear the difference from natural reverb sounds.
Which pedal should I get?
Many guitarists relies on the reverb in their amp, and there are many amps with great reverb. However, chances are that you're not playing your own amp at small club gigs or even bigger shows. Then it's great to bring a reverb pedal instead of taking a risk and end up playing an amp with a bad, or none, reverb. There's some great reverb pedals out there including the Electro Harmonix units Holy Grail (which comes in several different models) or Cathedral Stereo Reverb, the TC Electronic TonePring Hall Of Fame Reverb pedal or the Boss units RV-5 or FRV-1 63 (which emulates the sound of spring reverb units from the classic year 1963). Try some pedals out and find out which one you like the best.