The authoring of tags -- unlike the authoring of traditional metadata -- is highly popular among users. This harbours unprecedented opportunities for organizing content. However, tags are still poorly understood. What do they ''mean'', in what senses are they similar to or different from metadata? Different tags support different communities, but how exactly do they reflect the plurality of opinions,what is the relation to individual differences in authoring and reading? In this paper, we offer a definition and empirical evidence for the claim that ''tags are not metadata, but just more content''. The analysis rests on a multi-annotator classification of a blog corpus using the WordNet domain labels system (WND), the development of a system of text-classification methods using WordNet and WND, and a quantitative and qualitative comparative analysis of these classifications. We argue that the notion of a ''gold standard'' may be meaningless in social media, and we outline possible consequences for labelling and search-engine development.