Google Analtyics is a free online tool offered by Google that provides important information about your website or blog, including, but not limited to:

The number of visitors to your website or blog during any given time period.
Where your visitors came from – by directly typing in your URL, from a link, or from a search engine.
Which search engine keywords were used that resulted in a visit to your site
Which pages within your website or blog were visited, how frequently, the average length of each visit.

And much, much more.

Google Analytics offers everything from the basics to the very detailed and generates a variety of reports, which you can customize and even export to save on your computer or email to someone else.,

Google Analytics may also be used in conjunction with Google AdWords to track the performance of your pay-per-click ads and landing pages. Google Analytics can also work in concert with your ecommerce shopping cart to track sales for an online store.