Word describing nouns or pronouns can be restrictive or nonrestrictive. A restrictive element defines the meaning of the word it modifies and is essential to the meaning of a sentence. A restrictive element/clause is not set off with commas. Use that only with restrictive clauses.
Restrictive Example: For camp the children needed clothes that were washable.
If you remove the restrictive element from the sentence, the meaning changes significantly. It would only suggest the children needed clothes. The intent of the writer is to suggest the children needs washable clothes.
A unrestrictive element describes a noun or pronoun whose meaning has already been defined. An unrestrictive element/clause is off with commas or parenthesis, because it is nonessential information and if removed does not change the meaning of the sentence. Use which with unrestrictive clauses.
Unrestrictive Example: For camp the children needed sturdy shoes, which were expensive.
If you remove the unrestictive element, it does not change the meaning of the sentence.