An adjective clause is restrictive (also called essential) if it narrows down the word it modifies. It tells which one of the noun you are writing about. A restrictive adjective clause is necessary to the meaning of the sentence. It is not separated from the rest of the sentence by commas.
The players who are wearing the red uniforms are winning the game.
If we take out the clause, we won't know which players are winning the game. It's a restrictive or essential clause.
An adjective clause is nonrestrictive (also called nonessential) if we know exactly who is being written about without it. A nonrestrictive adjective clause is simply adding extra information. Nonrestrictive adjective clauses need commas around them.
Those girls, who have been friends for years, are all going to the same college.
Without the clause, we still know that those girls are going to the same college. The clause is nonrestrictive.
A proper noun is usually followed by a nonrestrictive clause.
Amanda, who is my best friend, is on the honor roll again.
Without the clause, we know that it is Amanda who is on the honor roll.
That versus Which
The relative pronoun that always begins a restrictive clause. That can be used to replace who, whom, or which in restrictive clauses, but many teachers prefer students to use that only with non-human antecedents.
The oranges that you need for this recipe are on the table.
The workers who built this bridge did a good job.
The relative pronoun which generally begins a nonrestrictive clause. It can begin a restrictive clause, but most style manuals prefer writers use it only for nonrestrictive clauses.
The oranges, which have been sitting on the table for a week, are starting to look brown.