What is tremolo?
Tremolo is a word that has been used and misused many times and therefore has quite different definitions. It's commonly referred to as many fast repetitions of a single note, a technique used on bowed instruments but also on the classical guitar which can be heard in pieces like "Requerdos De La Alhambra". A related english word that comes to mind is trembling. It can also be achieved on percussive instruments as a roll. Sometimes it's also used for a slow type of vibrato, especially when it comes to singing, and that is probably why a whammy bar also is known as a tremolo arm. But when it comes to guitar pedals, or amps with this feature, the effect is recognized as fast volume variations which creates a shuddering sound.
Any good examples?
The tremolo effect has been used in a lot of different context but it often stands out in alternative pop, Johnny Marr of The Smiths uses it well in "How Soon Is Now" and Peter Buck of R.E.M. utilizes it on "Monster" tracks "What's The Frequency, Kenneth?" and "Crush With Eyeliner". Other modern examples includes "Bones" by Radiohead and "Boulevard Of Broken Dreams" by Green Day, while older acts like The Rolling Stones and Led Zeppelin used tremolo on "Gimme Shelter" and "Down By The Seaside". Another fan of the tremolo effect is Tom Morello who employs it on tracks like "Like A Stone" (Audioslave) and "Guerilla Radio" (Rage Against The Machine).
Which pedal should I get?
Tom Morello, and a lot of other guitarists, uses the standard choice - the Boss TR-2. But there are other options, like the T-Rex Tremster, the Rocktron Pulse Tremolo or the EBS Triple Tremolo Controller. Each pedal comes with different special features (and prizes), so make sure to check out a number of them before you settle on which one you want.