Non-defining relative clauses, unlike defining ones, add extra, non-essential information, and are always punctuated with commas—a key difference impacting sentence interpretation and meaning. They're often used to elaborate on already identified nouns. The grammatical structure of non-defining relative clauses includes a subject, verb, and object, mirroring independent clauses but functioning as modifiers; they are easily identified by the surrounding commas and their supplementary nature to the core sentence meaning.