Okay, I am not embarrassed to show my ignorance...
what is considered a "unique visitor"?, as compared to just plain visitor?
If you check in on FR five times in a day, that is five visits. But you are just one "unique visitor" who visited FR five times. Another term is "page view" which is how many times you viewed a new page, which is larger than the number of visits. People looking to inflate their statistics will quote "page views" or "visits." But the true indicator of how many people are visiting a site are is the number of "unique visitors".; Which actually could be higher than the stats show because if both you and your wife visited FR twice (once each) using the same computer, the stats would show just on unique visitor since it is based on logging your computer's IP address.
We're all unique visitors. Some are just more unique than others. Take MurryMom for instance...
A unique visitor is a unique address (typically an IP address but usually bundled with other information in case everyone's coming from one IP). A visitor would be perhaps you but if they didn't break it down to unique then you may have 5-10 visits a day. A good unique algorithm will hopefully narrow your 5-10 to 1 'unique' visitor. However, it is actually impossible to get unique visitors unless you use authentication.
The problem with all of these numbers is that they need to be taken with a helluva grain of salt. Unfortunately, they are the best that is available.