There are essentially two types of Web metrics.
Basic metrics comprise the data that anyone can track easily.
But while basic metrics are easy to track, they're not all that useful. More advanced methods combine several basic metrics to yield more sophisticated and useful information, such as impressions and unique and nonunique visitors. But tying technical transaction data to actual people isn't very reliable. The most accurate way to gather visitor data is to make users enter a unique ID and password and then track their travels through your site with cookies or server objects. Unfortunately, many users resist IDs and passwords, so you could end up reducing the traffic you're trying to measure.