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It's an adverbial pronoun, along with "en". http://french.about.com/od/grammar/a/pron_adverbial.htm
"il y a" is informative. For example: "Il y a un chat dans la maison" means "There is a cat in the house".
"Voilà" is used to announce something that is just happening, for example: "Voilà un chat qui entre dans la maison" means "(And now) there is a cat entering the house", or when you give something to someone: "Voilà ton livre !" means "Here is your book!".