There are many types of online measurement systems available each with its own methodology and purpose. Confusion between these different types of systems is common and many system also overlap in there capabilities. The main groups of online measure systems are:

Traditional site stats package reporting centred on page views. Useful for reporting general stats like sessions, page views and referrers, also useful for accessing site usability and section/page popularity.
Examples include: Webtrends and LiveStats

Marketing software often come with its own measurement system that is tailored to one specific media. For example an email marketing solution will report unsubscribe, bounce, open and forward rates in addition to clicks. While banner serving software will concentrate on impressions, hovers, clicks and creative performance.
Examples include: TouchPoint and Double Click

These tools concentrate on measuring the effectiveness of all media used in a multi-channel marketing campaign and will combine traffic from banners, emails, links, affiliates and search to provide a common metrics frame work to evaluate the effectiveness of each media, campaign and creative against each other.
Example: Media Tracker

These tools report on the relative performance of one site to another and are excellent for competitor analysis or identifying market leaders in a particular category.
Examples Include: Red Sheriff and Hitwise

These systems use panel of surfers who are tracked to record their online activity. Similar to TV ratings these systems provide in-depth demographic analysis and supporting survey data.
Examples Include: Netratings