After you register with them, you are given some fictional Blog Dollars (B$) to go buy some shares in blogs.
Blogs are valued by their incoming links. Prices can go up or down based on trading and the underlying value of the blog.
You can buy as many as you can afford but up to a limit of 1250 shares in one purchase. You can also claim your own blog and will initially be given 20% of the shares. However, in order to do so, you have to place a claim button (actually a link button) on your site.
before you join in, I suggest you check out the Player tutorials here.