First of all, let's get one thing straight: It's tremolo.  It isn't tremelo.  There's no such word.  In spite of the fact that this isn't in the dictionary, it can be found in relative abundance on eBay.  A quick search turned up 464 hits under the guitar category for "tremolo" vs. 119 hits for "tremelo."  And I couldn't spell check this page without it giving me a heads-up that this was a nonsense word.  So there you go.

The odd thing about this term is that is has come to have three distinct and almost completely different, non-overlapping meanings.

• Hardware.  Primarily for most of us guitarists, it's the hardware that invariably involves springs and allows you to supply vibrato to your strings by mechanically tightening and/or loosening them around the original pitch.  A lot of people talk in terms of the "whammy bar," although this only works when it's attached to a tremolo.

• Effects.  It's also a guitar effect.  Rather than altering the pitch of the note, the tremolo effect on amplifiers and guitar effects pedals/processors alternately raises and lowers the volume of the output.  Guitarists like Duane Eddy frequently used this effect to give his guitar a menacing tone from a paradoxically "trembling" sound.  This is common on many country and western recordings.

• Picking.  Finally, it's also a style of picking.  Rather than altering either the pitch or the volume, tremelo picking is picking a guitar string very rapidly, perhaps on the order of 64th notes, much like that employed in playing a mandolin.  Eddie Van Halen used this approach quite a bit in his early days.  Rather than holding the pick as most of us do between his index finger and thumb, he instead uses his thumb and middle finger.  This changes the center of mass for this hand, and thus makes it easier to rotate rapidly.  Hence we get Eruption.