I think if you go through this section you will have more knowledge about what is google analytics and how to use it - http://www.google.com/support/googleanalytics/
You can know visual representation of statistics or data of your website about your visitors. Like how much visitors,countries and on which keyword they found your website etc. Rest newwebseo explained.
It comes from the name it is Google Analytics and I'm 100% sure that you can find all the info that you need (basic info ) in Google
You can use Google Analytics with Google Website Optimizer (GWO), which is a tool that helps determine the most effective design for your site. When a website optimizer script executes on your page, a _utmx cookie is written to the browser and its value is sent to Google Analytics. Now a days google analytics is more time saver and reliable for check whole site value and data with visitor traffic information.
Google Analytics is basically used for know the traffic or volume about your site, it is also used for know about goal you achieve for your perticular site.......it shows the exact volume about the keyword of site, so you can know that on which keyword you have to do promotion.....