As people visit your website they leave behind a footprint. In fact they leave behind and entire trail of footprints. Every page they view, every link they click and every product they read about creates a footprint.
These footprints are saved in a file somewhere hidden on your website. But don't try to read the file, it would be impossible to comprehend because it's a bunch of numbers, codes and file names. This is perhaps the most valuable part of your website but in order to understand the value you need to process all that information through a program.
There are many programs that do this, and popular programs like Webalizer and AWStats are included with most websites. A really fantastic program is Google Analytics. There's a little work with setting up Google Analytics, so ask your website guy to do it for you.
Make time to study and understand your Analytics reports. Don't bother creating a website if are unwilling to make time to learn this.