The search capabilities discussed above give users the ability to find a group of records in an ad hoc fashion, but there is no permanence to the results. Tagging gives users the opportunity to identify records in an ad hoc fashion by associating tag values with individual records. You use these tags to find groups of records with one or more tags.
Tagging is a familiar process on Web 2.0 sites. A tag adds another value to the metadata for a record that can be used to retrieve records with the same value.
Tagging functionality is built into the Force Platform, but is not enabled by default. As with search, you add tags to your application through the Customize menu.
Click Setup Customize Tags Tag Settings and select the Enable ...