As of today user tags are exported with the daily data base dump. They are in the file 'userTags.zip' with two tab separated columns : geonameId and Tag.
The tags are used to flag a place name. In contrast to the feature codes and feature classes which are one-dimensional (a place name can only have one feature code) several tags can be used for each place name.
It is an additional categorization mechanism where the simple classification with feature codes is not sufficient.