Keywords are the words that academics use to reveal the internal  of an author's reasoning. While they are used primarily for rhetoric, they are also used in a strictly grammatical sense for structural composition, reasoning, and comprehension. Indeed, they are an essential part of any language.
There are many different types of keyword categories including: Conclusion, Continuation, Contrast, Emphasis, Evidence, Illustration and Sequence. Each category serves its own function, as do the keywords inside of a given category.
Corpus Linguistic Key Words 
In corpus linguistics, key words are words that appear with statistically unusual frequency in a text or a corpus of texts. They are identified by software that compares a word-list of the text with a word-list based on a larger reference corpus. A suitable term for the phenomenon is keyness. The procedure used, for example by WordSmith, to list key words and phrases and plot where they appear in texts. These items are very often of interest—particularly those human readers would not likely notice, such as prepositions, time adverbs, and pronouns.
See also: Scott, M. & Tribbe, C., 2006, Textual Patterns: keyword and corpus analysis in language education, Amsterdam: Benjamins.