If you hear some Finnish word that sounds odd there's a good web page to help you out. Urbaani Sanakirja has these words that are used in street slang, but which you won't learn in your Finnish class. Mostly used between teenagers, but yeah - everybody do use some of these words in some way sometimes.
The web page is only in Finnish, so it requires you to know some Finnish. Some of the words are hilarious, like "mennä reisille" (free translate: it went tights) means that thing that you or someone did, didn't go well at all.