Enterprise Keywords and Metadata Publishing in Libraries
If you have the Managed Metadata Service application configured and enabled in Central Admin, you can make use of Content Type Publishing (share content types across site collections) and Taxonomy (hierarchical term sets that can also be shared across site collections). In addition to the managed term sets, there is the so-called Enterprise Keywords – terms that are stored in a non-hierarchical way and available for users to key in (folksonomy). If enabled in a library, these keywords can also be made available as social tags especially in My Sites. You can enable the use of enterprise keywords from the list or library settings, under the Permissions and Management section (the option: Enterprise Metadata and Keywords Settings). This options is disabled by default.
In libraries, you have the option to also publish these as social tags so they can be used in My Site and make it into tag clouds etc. This second option (metadata publishing) is not available in lists. You can still use the Tags and Notes to mark a list item (either privately for your own use or in public).
Note: there are statements that the second option is also available for lists.