Skyleebug, at the top of this page you'll see a blue tab "Help". Click on that, and scroll down the resulting page to the Frequently Asked Questions (FAQ) section. There's a topic header there that takes you directly to a layout explaining AS charges (they're cheap!).
When you block a person, they can no longer invite you to a private message or post to your profile wall. Replies and comments they make will be collapsed/hidden by default. Finally, you'll never receive email notifications about content they create or likes they designate for your content.
Note: if you proceed, you will no longer be following .