domainstats is an alias for org.apache.nutch.util.domain.DomainStatistics
In short its a tool which provides information about which domains have been fetched.
usage
$ bin/nutch DomainStatistics inputDirs outDir host|domain|suffix [numOfReducer]
example
$ bin/nutch DomainStatistics hdfs://nn:9000/user/otis/crawl/crawldb/current hdfs://nn:9000/user/otis/ds-host host 8
You can then -cat ds-host file from DFS and pipe it to sort -nrk1 for sorting by count, higher count first.