You don't need to know, the terms are essentially interchangeable, and the only people who care are people who you normally wouldn't want to associate with anyways, i.e. the classically educated music snob.
But why SHOULD you learn? Because at this moment, that very same music snob has a piece of information that you don't. Information that he/she can, and will, hold over you, making you feel inferior at their very first chance. It's time to fight back.
A great way to think about it, for guitarists especially, is that the main scale we use, the major scale, has seven notes. The notes are represented by seven letters of the alphabet, and other notes occur between these notes. For example:
A - A#/Bb - B - C - C#/Db - D - D#/Eb - E - F - F#/Gb - G - G#/AbNow when you think of a scale, pretend that every one of those letters needs to be represented in some way. Let's take D for example. It doesn't matter if it's D, D#, or Db, every key needs to have some kind of D in it. Now let's look at the notes in the key of E major.
E - F# - G# - A - B - C# - D# - EThe reason that the seventh note in that key is a D#, and not an Eb, is that the key already has an E, but if the seventh note were referred to as Eb, there would be no D note represented. Or look at the second note, F#. We could refer to that as a Gb, but then we'd have two Gs and no Fs.
One more example, the key of Bb:
Bb - C - D - Eb - F - G - A - BbThis is referred to as the key of Bb, and not the key of A#. If it were the key of A#, the seventh note would still be an A. Everyone in the world will know what you're talking about but some will say it's "wrong" to be in the key of A#.
In reality, referring to the second note in the key of E as a Gb would only be problematic if you were writing the notation out on a staff (you'd be trying to cram two different notes into the spot associated with Gs note), but there will always be some snob out there trying to correct you, even when they know exactly what you're trying to say.
It all comes down to communication, and musicians of different skill levels and philosophies have always had issues getting their ideas across to each other. So be the bigger person here and throw the classically-trained musician a bone. Odds are, they hurt inside and are deeply jealous of the ease we guitar players achieve glory.
By Sean Daniel