A bookmark in its simplest form is a saved URL. It can be tagged, put in a favorites list, or added to a list in your browser. When you start using a social bookmarking systems it is a little different.
Social bookmarking is a way to gather, organize, search, share, and manage bookmarks of web pages on the Internet.
People save links to web pages that they want to remember and/or share. This is usually done using a service like delicious. With services like delicious bookmarks are usually public, and can be saved privately, shared only with specified people or groups, shared only inside certain networks, or another combination of public and private domains. You can access bookmarks through different channels: tags, chronologically, folders, search engine, etc.
Tags seem to be the trend and most commonly used organization method for large amounts of social bookmarks.
Many social bookmarking services provide web feeds for their lists of bookmarks, including lists organized by tags. This allows subscribers to become aware of new bookmarks as they are saved, shared, and tagged by other users.
As these services have matured and grown more popular, they have added extra features such as ratings and comments on bookmarks, the ability to import and export bookmarks from browsers, emailing of bookmarks, web annotation, and groups or other social network features.
Wikipedia lists the following as social bookmark sites:
Balatarin (Iranian site)
BookmarkSync (private bookmarking, created to give you your bookmarks at any cpu)
CiteULike (for sharing scientific references)
Connotea (similar to CiteULike; for scientists, researchers, and clinicians)
Digg (voting on the bookmarked links sets this one apart)
Diigo (users can highlight parts of a webpage and attach sticky notes to highlights or to a whole page and these notes can be kept private, or shared)
Faves (content is supposedly a little more 'rateable' than delicious)
IBM Lotus Connections
Linkwad (for Mozilla lovers)
My Web (Yahoo's version)
Mister Wong (German startup that is big in Europe; similar to delicious)
Mixx (more of a social network with bookmarking as the social object, I think)
Newsvine (more like a social news network)
oneview (German and English started in 1998)
Propeller.com (used to be netscape.com)
Reddit (bookmarks appear based on # of votes)
SiteBar (available in more than 20 languages)
Windows Live Favorites (part of the Windows live application suite)
Many of these sites have specialties. As I searched some you could tell they had more bookmarks for media based sites. Others focused on politics and other on academic realms. Surely there would be a place for religous bookmark group...particularly one centered around things that could help people and answer their questions and that help get the type of content out on the web that is on mormon.org. How would you get that to spread to a circle of people that is not already LDS?