Watch the Modes Shmodes online guitar lesson by Angus Clark from Hard Rock Survival Guide: Lead

Modes? Modes shmodes! Modes are sounds created by altering certain degrees of either the major or minor scale. The major modes are Ionian, Lydian, and Mixolydian. The minor modes are Aeolian, Dorian, Phrygian, and Locrian (Locrian is really Dimished, but we're doing this in broad strokes, so give me a pass). Here's a map of what the formula for each is. For the purpose of this map the "#" and "b" and "symbols are used to indicate a scale degree being raised or lowered a half step from it's ordinary position:
Ionian = Major Scale
Lydian = Major Scale with a #4
Mixolydian = Major Scale with a b7
Aeolian = the Natural Minor Scale = the Major Scale with a b3, b6, and b7
Dorian = Natural Minor with a #6
Phyrgian = Natural Minor with a b2
Locrian = Natural Minor with a b2 and a b5

What's more important is to understand how they sound. I give some basic examples in the discussion in the video. Major sounds like classical music, or the hymnal. Minor sounds like Iron Maiden. Lydian sounds like Tom Petty or the Simpsons theme. Mixolydian sounds awesome, and kind of psychedelic and can be used over blues chords. Dorian is cool and a little jazzy. Phrygian is dark and very metal. Locrian is super dark, almost too dark. Like none more dark. Cool stuff.