You have a collection of assorted newspaper articles. You make a word list based on these articles, and see that the most frequent word is the. Among the rather infrequent words in the list come examples like hopping, modem, squatter, grateful, etc.
You then take from it a 1,000 word article and make a word list of that. Again, you notice that the most frequent word is the. So far, not much difference.
You then get KeyWords to analyse the two word lists. KeyWords reports that the most "key" words are: squatter, police, breakage, council, sued, Timson, resisted, community.
These "key" words are not the most frequent words (which are those like the) but the words which are most unusually frequent in the 1,000 word article. Key words usually give a reasonably good clue to what the text is about.
Here is an example from the play Othello.
See also: word-lists with tags as prefix.