Google Analytics is the enterprise-class web analytics solution that gives you rich insights into your website traffic and marketing effectiveness. Powerful, flexible and easy-to-use features now let you see and analyse your traffic data in an entirely new way. With Google Analytics, you're more prepared to write better-targeted ads, strengthen your marketing initiatives and create higher converting websites.

Source: Google.

Confused? Basically Google analytics keeps an eye on your website and lets you know how many hits your site has, where the people are that are looking at your site, which pages are popular, where people find your site etc etc. and you can you look in any time you like to check this data or receive a report via email when it suits you.