Non-defining relative clauses are composed of a relative pronoun, a verb, and optional other elements such as the subject or object of the verb. Commas or parentheses are always used to separate non-defining relative clauses from the rest of the sentence.
As the name suggests, defining relative clauses give essential information to define or identify the person or thing we are talking about. Take for example the sentence: Dogs that like cats are very unusual. In this sentence we understand that there are many dogs in the world, but we are only talking about the ones that like cats.