In 2003, Delicious (then called del.icio.us) was founded and coined the term social bookmarking. Even though the concept had been around for some time, Delicious pioneered the concept of tagging the links allowing for easy categorization of your bookmarks.
If you aren't familiar with the idea of social bookmarking you might be familiar with some of the websites that provide this service.
Today, arguably the most popular social bookmarking site is Pinterest. But, historically, depending on the audience, others that have been (and continue to be) popular include StumbleUpon, Digg and Reddit.