Dialyzer uses something called Success typing. The gist is that while e.g. Haskell won't compile unless it can prove that your code is correct, Dialyzer will only complain if it can prove that your code is wrong.

Personally, when I used Dialyzer in Erlang, I was underwhelmed by the amount of errors Dialyzer caught (type checking concurrent code is hard, who knew?) — however, it is certainly better than nothing. Plus, the type specifications you have to annotate your functions with help with documentation :)