A defining relative clause gives essential information about the noun or noun phrase it modifies, without which the sentence wouldn't make sense as the listener or reader would not be able to identify the noun in the sentence:
The hotel that we stayed in wasn't bad. ('that we stayed in' tells the listener which hotel we are talking about; it defines the hotel)
'Who', 'whose' and 'that' can be used for people. 'Which' 'whose' and 'that' can be used for things.
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