There are several metrics that are commonly found on all analytics programs. They count some basic information about your website. If you're going to only follow one or two metrics on your site, you should probably start with these.
Pageviews When you are tracking pageviews, you're tracking one of the most basic elements of your site. How many times have the pages on the site been viewed. A statistics package or analytics tool that doesn't provide this information isn't worth the name. Pageviews doesn't tell you how many people saw your site or give you much information other than how many times the pages were requested from the server.
Tracking pageviews is useful but it can be gamed. It's possible to see your pageviews inflated by automatic processes that really don't represent any type of person seeing your pages. While it might be interesting to know that a spider has hit your site, if your pageviews are not monitored to account for them, you may think your site is more popular than it is. I know of situations where spiders were giving websites double the amount of human traffic. Since all they were tracking were pageviews, that site was severely hurt by an advertiser taking an audit and adjusting those numbers downwards (and the respective pay).
Browsers and Operating Systems Knowing what browsers and operating systems your customers are using is essential to create a site that meets their needs. Most Web analytics packages provide this information as well as pageviews. If you know that most of your readers come to your site on a Macintosh using Opera, then you know you should be testing your site in Opera for the Mac. Even if you don't control the design of the site, knowing the OS and browsers can help improve your content. For instance, if you're writing for a video game site and most of your readers are on a PC, writing about Mac games is not going to interest most of your readers.
Knowing the browsers and operating systems your customers use can be misleading. For one thing, some browsers send false codes to trick publishers into thinking they are a standards compliant browser when they aren't. It's also possible to block the browser and OS information from being sent, or to send a completely different string. Finally, just because one browser is popular one month doesn't mean it will be the next month.
Referrer Referrer statistics are also very common in Web analytics packages. They tell you where people have come to your site from and what pages link to you. This is a standard item that is tracked in most Web logs. Knowing who is linking to you is a great way to learn how your site is viewed in the Web at large.
The problem with referrer information is that it's not always accurate. Just like with browser and OS, it is possible to send false referrer information to the server. People do this to protect their privacy and spammers do this to try and get more pageviews. Spam of this sort is especially common on sites that publish their logs. But even if you're just using referrers for your own use, you could be giving spammers more pageviews by visiting them.
Who is Coming to Your Site
Visits and visitors Another really interesting metric is to track who is coming to your website. Visits and visitors (also known as users, unique visitors, and uniques) tell you something about the people that are coming to your site. Rather than just knowing a high level number of times your pages have been loaded, visitors tell you whether it was 100 or 1000 people who were doing the loading. Visitors tell you how many unique people are coming to your site, and visits tell you how many times in a specific period they have come.
The problem with visits and visitors is that it's not very exact. Lots of tools use IP address to assign visitor status, but when a customer is using an ISP such as AOL they are on a dynamic IP. This means that if they come to your site today on IP "A", tomorrow they might come on IP "B". This would be tracked as 2 visitors each with 1 visit, even though it was really 1 visitor with 2 visits.
Another way to track visits and visitors is by setting a cookie on their machine. Then every time that cookie arrives and is tracked, you know that the same person has returned to your site. The problem is that many people are afraid of cookies or consider them a violation of privacy. And so they turn them off and refuse to accept them.
The important thing to remember, if you're going to pay attention to visits and visitors, is that it's not an exact number. It' an approximation. So if you're following your visitor numbers, you should watch for trends, not an absolute measure. In other words, set a goal to increase your visitors, but not a goal to get to 4,000 visitors.
Network information (speed, hostnames, locations) Knowing how fast your readers' connections are can go a long way towards creating pages that work for them. You can also use hostname and location information to target information to readers from specific areas of the country or on certain Web hosts.
Geographical location and language Some metrics programs can provide you with information about where your customers are coming from. While we're not yet at the point of having location data and geotagging on all our customers, there is a lot of information available about IP addresses that can be used to pinpoint where a cutomer is coming from. This can be especially interesting if you are targeting an international audience or a specific region of the world. Watching the geographical location in your metrics can help you determine if your campaigns are working.
Geographical data has a lot of the same drawbacks as visits and visitors. It's not automatically encoded into the information sent by a Web browser, and when it is it can be modified. There are metrics companies that specialize just in geographical tracking, but this can get expensive if you want extremely accurate data.