Litotes

(n.) [Light-tuh-tees] Figure of speech using an understatement, and expressing a strong statement while negating its opposite.

Note: This sounds confusing, but it's really not that bad.

Here are some examples:
"When I stepped on her toe, my wife wasn't a little upset." (i.e., she was VERY upset)

"When I broke his window, he didn't see it as a small problem."

Link here to here the pronunciation, (the emphasis is on the first and last syllables): http://www.thefreedictionary.com/litotes