restrictive / nonrestrictive clauses
A clause that identifies the noun vs. a clause that adds information about it.
A nonrestrictive clause adds information about a noun the sentence has already identified, and it takes commas: The night clerk, who arrived late, was turned away. A restrictive clause identifies which person or thing the sentence means, and it takes no commas: The applicants who arrived late were turned away. In careful writing, a restrictive clause begins with that for a thing and who for a person: the car that failed, the clerk who left. A nonrestrictive clause begins with which for a thing and who for a person: her car, which failed; the clerk, who left. That never begins a nonrestrictive clause. Other names for the pair are essential and nonessential, and defining and nondefining. See also comma with nonrestrictive clauses.
Examples
UseCars that fail the test stay in the lot. (the clause identifies which cars stay, so it is essential and takes no commas)
UseHer car, which failed the test, stayed in the lot. (her already identifies the car, so the clause adds nonessential information and takes commas)
UseMy brother, who lives in Ohio, is visiting. (my brother already identifies him, so the clause adds nonessential information and takes commas)
AvoidCars, that fail the test, stay in the lot. (cars alone does not say which cars, so the clause is needed to identify them; the commas wrongly mark it as nonessential, and that never takes commas)