A unique visitor is counted each time a unique IP address visits a particular domain or website. This means that for the most part each time a unique person visits your website they will be counted as a unique visitor. On the otherhand, a hit is counted each time a request for a file from the web server is made. A single webpage can be made of countless files, while some are made up of just one. Regardless, the number of hits will add up much faster than those for visitors, unless, of course, it is a domain or website consisting of a single webpage consisting of one file. Then the numbers would pretty much be similar. This is usually not the case, however. One million hits can be generated from a single visitor. Both are important but the only way to calculate the number of people visiting your site/domain is to track the number of unique visitors.
