These are modes, in the key of G. There are 2 more, the Phrygian, and Locrian, but they're used a lot less than these 5. Modes are like scales, each having it's own set pattern of whole and half step notes, and each it's own sound.. To fully explain it would take more space tha allowed on this page, so we'll save that for later. But to start with, the ionian mode is a major mode, having the same notes and sound as the diatonic major scale, the dorian is a minor mode, with a jazzy feel, the mixolydian is a major mode, and differs from the ionian by just one note, the b7, and is used in blues and jazz. The aeolian is another minor mode, having the same notes and sound as the natural minor scale, and the lydian is another major mode, differing from the ionian by just one note, the #4.