AXS is tracking software, written in PERL it's a CGI program that will keep track of your visitors, where they came from, and where they travelled throughout your site.

This is invaluable information to make changes to your site and to know what is working and what is not. It's always nice to see what someone searched for, where you were in the listings, and what engine they came from. A typical search log will hold all this information and more about the user arriving at your site. You'll find what browser they used, what screen resolution and a lot more valuable information.

Google Analytics, is something relevantly new, and is very good. It's free as well. In Google's normal way, it provides excellent information and is above par in usability and information provided.