A social bookmarking tool is a website that allows people to organize their bookmarks and tag them according to their content. Social bookmarking tools have been popular for years, and this is in part because of the unique communities that come into existence around the specific tags and notes that are associated with each one. Tagging is a very important feature of social bookmarking sites, and it was made popular by Delicious, a site that was launched in 2003. It helps to keep things organized in a way that frequent users of the bookmark lists will find useful to them, and it makes it easier to find things that are related to what they are interested in.
In fact, because of tagging it is very common for a social bookmark tool or website to, for all intents and purposes, create their own language that is only really relevant to the community that uses it. This language or lingo is referred to as a folksonomy, and because this shared vocabulary is only really useful to those that use the site, it makes the community much more tight knit and almost exclusive.
The tagging fascination has also spread to social media tools as well, which is why you find the option on social media sharing sites to organize things so that people looking for things with similar interests will be able to find what you post.