17号考，问that和which的区别？

语法上说that是限定性从句，which是非限定从句，但是我一直搞不清楚具体的含义：什么是限定？什么是非限定？

恳请nn帮个忙！

请看OG11-07上的一段解释：

The pronoun which introduces nonrestrictive clauses, which include information relevant but not critical to an understanding of the sentence.

The pronoun that introduces a restrictive clause, which is critical understanding the sentence because the clause defines its antecedent.

也就是which删了不影响句意，that删了就不成了