I got down all the Authentic Modes years ago, and I came across an article talking about the Plagal Modes, and I started to do some reading on the subjet, just when I thought I understood everythig...I found other modes; so now, I have to investigate about those Modes and see what the differences are.
So these are the modes to compare:
Dorian-Hypodorian-Hyperdorian.
Phrygian-Hypophrygian-Hyperphrygian
Lydian-Hypolydian-Hyperlidian
Mixolydian-Hypomixolydian-Hypermixolydian.

The term "Hypo" means playing below, so Hypodorian is playing below the Dorian mode.
The term "Hyper" means playing above, so Hyperdorian means playing above the Dorian Mode.
But, the rules regarding the use of those modes, are very strict. (i.e. what cadences belong to each mode, intervals to avoid, etc)

Are you going crazzy yet?