Re: For guitarists of all levels...
If you were asked to play D Ionian, you would play D major.
If you were asked to play D Aeolian, you would play the notes of D minor in the key of F (with a B flat).
The underlying harmony could be in any key, but you wouldn't care about dissonance.
For example, the harmony could be in D major and the melody in D Mixolydian - which would include a C but not C#. This is quite common and creates a bluesy sound on the song. The flavour of the melody is different from playing the same notes as in the D major scale.
The difference between the two is nomenclature (the difference between Sodium Chloride and Salt), and serves as a way to express the sound other than the names of the notes. You know that when you play Aeolian mode over a major scale of the same note, you get a dissonance at two points (figure out which
). This dissonance creates texture.
A scale is built on intervals. A mode is a convenient way of expressing how that scale would sound against the harmony underlying it - the emotion of the music.
Clear as mud, no? The best way to figure out modes is to play through them over a harmony. Use a jamtrack or have a friend play chords in the key of D. Then play through each mode up and down, and you'll begin to get a feel of their power.
Note that modes are used by very few people. Most modern music is strictly pentatonic, or at most full-scale. Blues scale is also popular, with a flattened seventh and a few other variations. I can think offhand of Satriani and Steve Vai as users of modes in some pieces (specially the latter), but struggle to think of any recent music that resorts to using modes. Most listeners don't have the time. You may be able to find modes in Metal music, but I don't listen to it so can't comment - however it's quite likely.
hyeah:
