User-based tagging allows users to organize resources by assigning keywords or tags. Tags are used to group related resources together so that all items with a particular tag can be discovered at once. When many non-professionals or "folks" contribute tags it creates a folksonomy, where individual tags are combined into a tag cloud that can be accessed by multiple users. Popular sites for user-based tagging in libraries are Delicious, LibraryThing, and PennTags, which allow users to organize and share bookmarks, books, and other resources.