Hi Guys, this is probably a "really" stupid question and one that has most likely been addressed before. But what does Google base the page ranking system on, is it visitor related or links to your site that matters most? I've recently gone up to a 7, but am not sure how I got there

Traffic has only slightly increased - but link backs into the site is already increasing steadily. So hence my question on what it's based on.
Thanks in advance!
Karen