Unique Visitors Since 3/30/09
Let me explain what a "Unique Visitor" is. It's tuff to tell as visitors is based on cookies which some people disable and some people often clear.
Returning Visitors - Based purely on a cookie, if this person is returning to your website for another visit an hour or more later
First Time Visitors - Based purely on a cookie, if this person has no cookie then this is considered their first time at your website.
Unique Visitor - Based purely on a cookie, this is the total of the returning visitors and first time visitors - all your visitors.
Page Load - The number of times your page has been visited. (My page load total is almost double the Unique Visitor Count)