What's the difference between directory & social bookmarking?

Directory submission helps to make the backlinks & social bookmarking helps to get the traffic to your post as well as also help to get the backlinks for your website!
Directory submission is to create one way links to a website.

While Social bookmarking is a method for Internet users to organize, store, manage and search for bookmarks of resources online.

Directory link Is helpful to increase back link and PR, social bookmarking is quite helpful to provide traffic.