You can easily add your web bookmarks, label them with as many tags as you like, and access them whenever you’re online, at home, at work, wherever. Handy when you have a number of machines, and have trouble remembering links from one to another.
That’s the ‘Bookmark Manager’ bit. The ‘Social’ bit comes from the fact that you can see how many people have also linked to the same things, what tags they’ve used to describe them and what else they’ve linked to. And that’s it really. There’s all sorts of addons and plugins for the better browsers like Firefox, and tools to help you import any existing bookmark lists, but the basics make it what it is – simple, useful and fun.
There’s loads of info on the web about it – Google has them.
Edit 2: Not cool when it’s broken.