Let's say you have a database that is keeping track of every visit to your website.  It tracks the IP address and the time.  Naturally, in that database, you'll have the same IP address multiple times from repeat visitors.  If you were to do a normal count(*) on that database you would get a number representing all non-unique visitors.  It would include the person that visited your site multiple times as multiple visits.  To find out the number of unique visitors you would use a special directive in MySQL called 'DISTINCT'

Therefore our query would be: {codecitation class="brush:sql"}SELECT count(DISTINCT ip) FROM visits;{/codecitation}