One of the most important aspects of website is traffic analysis. If you do not know where your visitors are coming from and in what numbers you cannot effectively promote your site, or gauge the effects of any current promotion efforts. Checking the stats for your site(s) should be a daily activity, and if you're not doing it you need to start.
There is some confusion as to the different terms used to describe website traffic, misuse of these terms often causes miscommunication so it is important you know the correct words for what you are talking about. The definitions below are of the most common terms you will find:
Impression: A page view. An impression occurs when someone views one of your HTML pages. If you use frames though you should only count impressions on your main content pages, not those on the pages you use for your menu or header frames. Another way to look at this is to just count impressions on pages that have advertising on them.
Unique: A page view by a unique person within a 24 hour period. Uniques are usually measured using the IP address of the visitor using your site. However some services, notably AOL, send all their members through proxy servers so that thousands or millions of people can share the same IP address. This usually means that if you are recording the number of uniques by looking at impressions by unique IP addresses then your actual number will be slightly higher than what is reported. A better way to measure uniques would be a composite unique value composed of IP address, browser or user agent, and operating system.
Referrer: A page which links to your site. This doesn't have to be an actual page, it can be the result set of a search engine. Looking at your referrers is how you know who is linking to your site.
User Agent: Software used to access your site. Also sometimes known as browser or client a user agent can be a PHP script, a browser like Internet Explorer, or a search engine spider like GoogleBot. Knowing what software is used to access your site can tell you if someone is abusing your site or when search engines last crawled your site.