Since Facebook launched their share link feature anyone including you can share your site or pages to their friends. You can tailor what is displayed on the Facebook wall in terms of title, description image and URL.
Facebook is smart enough to use the html meta tags on your page for title and description and it may list some or all of the <img> tags to the user to choose from. This is great as a starting point and may be enough in many cases, however in some it is not.
You find all detail on PureSourceCode