The document defines parallelism as the similarity of structure in related words, phrases, or clauses. It provides examples of parallel structure using nouns, verbs, adjectives, and adverbs. The document also discusses the five main ways writers use parallel structure: with elements joined by coordinating conjunctions; in lists or series; when comparing elements; with elements joined by a linking verb; and with elements joined by correlative conjunctions. It provides additional examples and exercises to identify and fix errors in parallel structure.