Your user account comes with a free blog. You can write anything in your blog. Popular uses might be writing about your school, personal road trips or car outings.
A new page is published for every blog entry and the title and first few sentances of the blog are listed on the main blog page. If you have entered your Google ID in your user account then the advertising is using your ID and the user that referred you..
A list of people you referred and earned referral points from can be seen here when you are logged in.