A nonrestrictive clause is also called a nondefining clause.
A nonrestrictive clause looks to the noun modified and adds information about it. It does not single it out among others that could exist in the context. Instead, the identity of the modified noun is either already known from context or is known because there's just one of those things named by the noun.
You should introduce nonrestrictive clauses with the relative pronoun which (unless who, whom, or whose is appropriate). Commas must set off the clause from the rest of the sentence.
Following are some examples of nonrestrictive clauses. It is assumed that, from context, the reader would know the identity of the thing, person, or idea named by the noun. The clause is not necessary for identification. It is, in a word, nondefining.
My best friend, who lives next door, came to dinner. The U.S. Supreme Court, which is across the street from Congress, decided the case. The book, which arrived in the mail, enraptured the entire family.
See restrictive clause.