A verb is a word that describes actions, occurrences, or states of being and is essential for sentence structure, with every sentence requiring at least one verb. Verbs can change form based on subject, tense, mood, and voice, which is known as conjugation, and they must agree in number with their subjects. The document also discusses different types of verbs, including regular and irregular verbs, and explains concepts such as verb tense, active and passive voice, and participles.