If you have a host worthy of giving money to, you should be able to view some type of statistics on who is visiting your web site. The three terms above are often confusing to people looking through their web site statistics for the first time.
A hit is simply a request for a file from your server. For example, when your browser requested this file my server logged a hit to /hits-visits-uniques (look in the browser bar, that's the file you opened). Were there a graphic on this page that file being requested would result in an additional hit.
A visit is just that, someone who visits you. Your web server keeps statistics tied back to the IP address of the viewer. So it's relatively simple for your statistics program to find out that the 21 hits in the portfolio example above all came from the same computer, which means 1 visit.
Unique visits is equal to the number of unique IP addresses that visit your site. For example, you are viewing this page now and you decide to come back and view it tomorrow. That would register as 2 visits but only 1 unique visit.
And some extra notes
It seems so simple, but there are a few extra things to keep in mind.
Unqiue visitors aren't always unique. AOL and other large ISPs often use a pool of IP addresses to visit pages. So you may get visits from 5 IP addresses, but it's really only one visitor. Most visitors are also coming from IP addresses that change every time they connect to the internet. So a visit from two different IP addresses (2 unique visitors) may really be one person.
And you may have more visitors that your statistics show. Large ISPs often keep cached copies of web pages on their servers to allow faster browsing by their clients. In this situation, only the first page view will be logged by your server.