hyperbole
A figure of speech (a form of irony) in which exaggeration is used for emphasis or effect; an extravagant statement
The language of hyperbole amplifies reality by carrying us beyond the boundaries of rational thought . . .
Examples:
Your mama's hair is so short she could stand on her head and her hair wouldn't touch the ground. . . .
Your father is so low he has to look up to tie his shoes.
You're so low down you need an umbrella to protect yourself from ant piss.
http://grammar.about.com/od/fh/g/hyperboleterm.htm
Understatement
A figure of speech in which a writer or speaker deliberately makes a situation seem less important or serious than it is.As such, understatement can be used to reflect modesty, sarcasm, derogatory or complimentary tone.
Examples Of Understatement
"It's a bit yellow" - while describing a very yellow canary.
"There is some music by Beethoven in his Ninth Symphony" - while describing Beethoven's famous work.
"The desert is sometimes dry and sandy" - While describing the driest desert in the world.
"It is just a little cool today" - when the temperature outside is 5° below zero.
"The food was tolerable" - on the food that was prepared by the best chef in the world.
http://grammar.about.com/od/tz/g/understateterm.htm