A hit is every request to a web site. A single page may generate many hits: each image is a hit, for example, as is the text, the .css call and so on.
A page view is the presentation of a web page to a user. It requires anything from 1 to a bazillion hits.
A session is the time an individual visits a site for.
A unique user is an individual visitor to a site. An individual may visit a site one or more times over the course of the measurement period, typically a month. Compare with the total number of users, which is effectively the total number of sessions. Since UU's the closest advertisers with a "magazine" mentality can get to the site's "circulation", it's required for ABCE audits. But it's no use for planning site capacity, for example.
In reality, things are quite vague: http is a stateless protocol, so if you only use site logs to generate statistics, it's difficult to determine when a session ends. If a visitor leaves your page open in a tab all day, and clicks on it once every 29 minutes, it's difficult to decide whether that's one session or many. Similarly for UU's: by convention, you need an IP address plus one other piece of data (cookie, etc). With AJAX, where parts of a page can be changed without refreshing the entire page, it's hard to decide when it stops being the first page view, and starts being the second. There are lots of techniques for improving the quality of site statistics.
There are many other metrics, including site depth and site bounce. In reality, you use them for two purposes: to plan capacity, and to satisfy advertisers. In the latter case, most advertisers trust their own data more than yours.