Modifiers and clauses that follow the words or phrases they modify are either restrictive or nonrestrictive. Restrictive modifiers and clauses are essential to the meaning of the sentence; without them, the meaning would be different. Nonrestrictive modifiers supply additional information; without them, although there would be less information, the basic meaning would be the same.
Not all the galaxy's stars are confined to the galactic plane. There are
a few stars that occur far above or below the disk.
--"Astronomy," Compton's Encyclopedia
S. C. Johnson Corporation, which makes Johnson Wax and
Raid, the antiroach product, breeds 80,000 roaches
a week and plays host to up to a million roaches at any one time.
--"Roach Wars," Scientific American
The distinction between restrictive and nonrestrictive modifiers is very important for punctuation: only nonrestrictive modifiers are separated by commas from the noun phrases they modify. See Nonrestrictive Modifiers.
The distinction between restrictive and nonrestrictive modifiers is also important when choosing between the relative pronouns that and which: in American English, generally use that with restrictive relative clauses; use which with nonrestrictive relative clauses.