Friendship books (also known as "FBs" in their abbreviated form) are small booklets made by stapling paper together, or are sometimes just sheets or strips of paper. They are usually decorated and the person who starts the book writes their name and address as the first person sending the book. People often include a list of interests as well. The FB is then passed around from penpal to penpal, and can often also become a way for one to meet new penpals. Most people hope to see the book again once it is full so they add their return address to the back cover of the book too, or inscribe "Return to Sender" on it. People also sometimes make FBs for someone else rather than themselves, in which case they write the name and address of the recipient on the front/at the top. Some people find friendship books fun because you can see where they have been in a trail back to the original sender/recipient.
This is the very first fb I have ever created. I love the fun and colorful style that I used for it. It just makes me happy to look at. Now if I can only get up the courage to send it out.