The visitors column will log a visitor more than once per day. The unique visitors column will not. Pageviews is a simple count of how many webpages are viewed in a month collectively. Hits is really only useful for looking at ways to optimize a page. A hit is logged everytime an element of a page is downloaded (each jpg, style sheet, html file, javascript file, media file, and so on count as a hit).
And of course there is always some portion of over-calculation on stats because of clients who surf through proxy servers and the like, but awstats does a rather good job of minimizing it.
Thanks for the info Harley, is there a section in awstat to check for unique visitors?