[Haskell-cafe] How is laziness defined?

I would think that with 100% laziness, nothing would happen until the Haskell program needed to output data to, e.g. the console. Quite obviously that's not it. So how is laziness defined in Haskell? I remember vaguely someone saying that pattern matching on a value forces it to be evaluated. Is that right? What else? This is one of the things that just boggles my mind everytime I try to wrap it around this thing called Haskell ;) Cheers, TJ