From their website:

Clustify analyzes the text in your documents to group related items into clusters. During that process, it identifies the most important keywords that cause the documents in the cluster to be considered similar to each other. The keywords give you a quick idea of what each cluster is about, and they allow you to easily identify the themes of your document set.

Clustify reports the frequency for each keyword, and allows you to browse clusters containing a set of keywords you specify. Clustify also identifies a “representative document” for each cluster. All documents in the cluster are similar to the representative document within the limit that you specify. This allows you to make decisions about all of the documents in the cluster by looking only at the representative document, reducing the amount of labor needed to review the documents…