Google Analytics is a free tool offered by Google to help you analyze your website traffic. Google Analytics gives you a short piece of code to place on your website that records various activities of your users when they visit your website. Things such as age, gender, interests, which pages they visit, how long they stay on those pages and which website they were referred from are all provided by Google Analytics.
All of my clients have Google Analytics installed on their website. Below are a few resources you can use if you're interested in learning more about Google Analytics and how to use it.