In addition to what longislandmark said,
Try taking a look at these links :)
http://support.chess.com/Knowledgebase/Article/View/16/0/how-do-ratings-work
How does blitz rating work?
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Initially I thought the blitz rating was a rank, and so was surprised to see the number increase whenever I won. How does the blitz rating work? What are the rules according to which the rating goes up / becomes better?