What is social bookmarking?Social bookmarking is a technique for managing the URLs of your favorite websites with an online service and being able to share these bookmarks with others. In contrast to managing bookmarks with a browser's built-in features, your bookmarks not necessarily need to be private to yourself, but others can browse through your bookmarks as well. Vice versa, you can browse through other people's bookmarks and discover new, interesting sites.
How are these services distinct from a browser's built-in feature?Your bookmarks are not locked into one browser or even one device (if a browser doesn't support cross-device synchronization): Depending on the social bookmarking service, you can access your bookmarks from any device, even on the go via mobile websites or mobile applications.
Is social bookmarking a new trend?Definitely: no. Social bookmarking is nothing new: The first online services for managing bookmarks were launched back in 1996. According to Wikipedia, itList was one of the first services for managing and sharing URLs of interesting websites. However, the term "social bookmarking" became popular when Delicious (del.icio.us) launched its service in 2003. They were one of the first services that established "tagging", a feature for adding keywords to items (here: bookmarks). Tagging was considered so useful that many other websites such as Flickr integrated it into their services.
How many social bookmarking services exist?Well, that's hard to say. At the time of writing, Wikipedia lists about 25 active services. I guess this list is by far not complete. This page lists more than 50 social bookmarking websites with German content. I think there are way more than 100 social bookmarking services in total.
What is the difference between all those social bookmarking services?
Some focus on simplicity and core functionality: They let you manage bookmarks and share them. That's it. Services such as StumbleUpon go much further and provide a complete browser-in-browser experience for discovering random new websites listed in profiles of people with similar interests.
Which services should I choose?
This question can't be answered easily. It depends on your personal requirements and, well, taste, which services is best suited for you. Several months ago, I was faced with this decision as well. I checked about ten different bookmarking services and none of them really convinced me. I wanted a service that allows me to focus on managing my bookmarks primarily, in a really simplistic way. Social sharing and the discovery of new bookmarks was secondary to me, however, relevant as well. Using the service on the go with my smartphone was another requirement. I found no such service and decided to create my own. It's called Cluefeed and you may feel free to give it a try.