The Companion contains precise definitions of words, people, places and events that have appeared in at least one Discworld novel, map, diary or non-fiction book, or in one of the three short stories "Troll Bridge", "Theatre of Cruelty", and "The Sea and Little Fishes".

Material is often quoted directly from these sources, but, in each successive edition, also includes information that had not yet been worked into the novels.

For instance, William de Worde is mentioned in the first edition of the Companion six years before the publication of The Truth, the novel in which he is introduced.

The book includes an introduction by Stephen Briggs and an interview with Terry Pratchett, both of which have been updated in each edition.

The book also contains a ten-page interview with Pratchett under the heading "Discworld Quo Vadis?".