A small piece of JavaScript placed on a website that sends data to Google Analytics. The tracking code is what enables Google Analytics to report information about your audience, including how they got to your website, how long they stayed, and what they did while they were there.

Building a Highly-targeted Opt in List: When Money Lies Beneath the Lists
Are you successful with your Internet business? Better yet, do you think you are successful with your online business? When most online entrepreneurs are asked with these questions, they often brush it off.