1. A method to refine a plurality of tags, comprising: receiving, by a tag refinement system, a plurality of tagging actions each of which associates one or more of the plurality of tags with a digital object; and refining the plurality of tags to reduce redundancy among the plurality of tags by extracting, by the tag refinement system, a first subset of tags from the plurality of tags, wherein the first subset of tags have a higher collective relativity-similarity score comparing to a second subset of tags selected from the plurality of tags, the first subset of tags, different from the second subset of tags, have a same predetermined tag count as the second subset of tags, and the extracted first subset of tags provide a representation of the digital object, wherein the extracting the first subset of tags further comprises generating a relativity-similarity score for a first tag selected from the plurality of tags based on (1) a relativity score associated with the first tag and (2) a similarity score associated with the first tag and a second tag selected from the plurality of tags and having a co-occurrence relationship with the first tag.