The 7 modes

• Ionian — the major scale. Bright, resolved.
• Dorian — minor with a major 6. 'Hopeful minor.' Use over m7 vamps. ('Eleanor Rigby', 'Oye Como Va')
• Phrygian — minor with a b2. Spanish, flamenco, metal. ('Wherever I May Roam' chorus)
• Lydian — major with a #4. Dreamy, floating. ('The Simpsons' theme, lots of film scores)
• Mixolydian — major with a b7. Bluesy, country, dominant. ('Sweet Child o' Mine' verse, most blues solos)
• Aeolian — the natural minor scale. The default 'sad.'
• Locrian — diminished tonic. Almost never used as a key, but matches m7b5 chords.

How to actually use them

Don't think 'I'll play Mixolydian.' Think 'this song is in G but feels bluesy — the F natural sounds right, so I'm in G Mixolydian, which is the same notes as C major.' The parent-scale view is fastest. The modal-tonic view (treating G as the new home) is more expressive but takes more practice.