From Nexus Wiki
To explain the usage of file tags on the Nexus sites.
Due to the varying complexity of mods, some mods benefit from being marked by tags, which may mark the mods as belonging in several categories besides the main one the author selected. "Files can be tagged by both file authors and logged in members; if an author tags a file it becomes instantly tagged where as 3 normal members need to approve a tag for a file before it becomes confirmed. A confirmed tag allows that file to be found using the tag search" 1. When a tag is submitted for approval by 3 normal members, those members also benefit from 3 activity points.
Links to the tag search
1. New Vegas Nexus. (2011). "File tags and you". Retrieved March 1, 2011.