Understanding your visitors is key to running and growing a successful website.
You need to know: Where They Came From
What They Looked At
How Long They Stayed
Where They Left From
Knowing exactly how your visitors interact with your website provides the insight necessary to improve conversions and develop content targeted at your audience.
This isn't the "Be All, End All" guide, but it's enough to provide a solid understanding of your visitors and provides a concrete foundation to using the Google Analytics tool.