Defining relative clauses...

• add essetial information about the nouns, and
• can be introduced by the relative pronoun that in informal speech.

Non-defining relative clauses...

• add extra information about the noun,
• are enclosed by commas,
• can be used with quantifiers and numbers followed by the relative pronoun,
• can't be introduced by the relative pronoun that, and
• can refer to a whole sentence.

Two types of relative clause:

• defining: give the information that directly identifies what is being talked about,
• non-defining: add more information about nouns, do not identify the noun being talked about.

https://en.oxforddictionaries.com/grammar/relative-clauses

Relative pronouns indtroduce relative clauses.

• who
• whom
• whose
• which
• that
• where: '(places) in/at which'
• when: '(times) on which'
• why: '(reasons) for which'

https://learnenglish.britishcouncil.org/en/english-grammar/pronouns/relative-pronouns http://dictionary.cambridge.org/grammar/british-grammar/pronouns/relative-pronouns