Google Analytics is great for assessing people’s interaction with your site, the fact that it’s a free tool is even better. Probably the most interesting/useful metric that it exposes is where people come from, both the physical sense with Google’s excellent geo-location and also on the web. In the case of the latter, it’s possible to break this down even further if you have a blog or send email newsletters by tagging your urls.
Analytics tagging is adding a couple of additional parameters to your urls that are picked up by the GA tracking code. These then show as additional sources/campaigns in Traffic Sources section of Google Analytics.
These are most useful for email newsletter tracking to help gauge how effective your marketing has been. Answering questions like did the copy in the newsletter engage people to click through, of those how many bought something or signed up for your service? Try putting different copy/design into some A/B testing to gain some insight into how well your layout or copywriting is working.