the modes all have a different sound to them.

Ionian is the major mode - it has a happy sound, and is used in rock music, folk music, etc.

Aeolian is the saddest mode. it is EXTREMELY sad, to the slit your wrists point. mainly used in very slow songs.

Phrygian either sounds dark, or spanish. it is used exclusively in spanish music.

Mixolydian has a very bluesy sound, used in blues songs, or melodic rock.

Lydian is much like the Ionian mode, but has a few different notes. it sounds happy, but has some "exotic" sounding notes, which gives it a sort of mediteranean feel.

Dorian is a sad mode. not nearly as sad as Aeolian, just slighly sad. mainly used in slow rock songs

Locrian, is hardly ever used. you dont have to worry about it unless you are a hardcore theorist.

i am speaking here from a guitarist perspective, but the rules apply to all instruments.

i have a very good MPEG video, by Frank Gambale. he explains modes and then gives examples of all the modes in use. it lasts 100 minutes.

if you have broadband, go to http://www.suprnova.org and do a search for guitar lesson, choose the frank gambale MODES lesson.