In B&W photography, filters are a fantastic tool to manipulate your results. You can use different kinds of colours to blacken or whiten your subjects.
I suggest you do some reading on this, as it's too elaborate to describe here... There are plenty of great books out there. For some incredible B&W photography, search in google for the name "Ansel Adams" (perhaps you've heard about him).
[Edited 2004-03-09 22:01:45]