A sentence is a group of words that expresses a complete thought. It typically contains a subject, which tells who or what the sentence is about, and a predicate, which tells what the subject does or is. For example, in the sentence "The dog barks," the dog is the subject, and "barks" is the predicate. Sentences can be classified into different types based on their structure and purpose. Declarative sentences make statements, interrogative sentences ask questions, imperative sentences give commands, and exclamatory sentences express strong emotions. Each type serves a unique function in communication.