A grammar is a set of rules that governs how words are combined to form sentences in a language. It includes guidelines for sentence structure, punctuation, and word usage, helping speakers and writers communicate clearly. Different languages have their own unique grammars, which can vary significantly. There are two main types of grammar: descriptive grammar, which describes how language is actually used by speakers, and prescriptive grammar, which sets rules for how language should be used. Understanding grammar is essential for effective communication and helps in learning new languages.