That's a question that many science blog-posts examine, implicitly if not explicitly. I do it here (quite a lot, recently!) when writing about various pseudo-scientific claims, & the same goes on over at Sciblogs NZ. It's an interesting question that sometimes receives rather slick, glib answers.
Over at Science-Based Medicine, Kimball Atwood has written a particularly good discussion around the 'what is science?' question, including an explanation of why many of those glib answers are based on a mis-characterisation, or a mis-understanding, of the nature of science.
(I really enjoyed reading it. But now - I must get back to revising my study guides!)