From their site: Extends the idea of social tagging systems like del.icio.us, Flickr or BibSonomy by introducing the group dimension. The foundation of social tagging systems are folksonomies. A folksonomy is just a collection of tag assignments. Over time it is likely that semantics emerge--tags that are often assigned to the same resources may be synonyms. With GroupMe's approach of taggable groups we extend tag assignments with a group dimension.