preventing automated attacks on my web sites guest book

Question

I often see an image with some wavy / distorted letters that the user must enter before anything can be posted to the database.. but don't know how to do this.  Can anyone help?

I want to put up a site with a guest book, and want to prevent automated attacks that could quickly fill my database.

Any help appreciated.  Any good books, tutorials, advice, etc. etc. etc. would be great.