Keywords are the words that are used to reveal the internal structure 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.