Relative pronouns introduce relative clauses. The most common relative pronouns are who, whom, whose, which, that. The relative pronoun we use depends on what we are referring to and the type of relative clause.
who

people and sometimes pet animals

defining and non-defining

which

animals and things

defining and non-defining; clause referring to a whole sentence

that

people, animals and things; informal

defining only

whose

possessive meaning;

for people and animals usually; sometimes for things in formal situations

defining and non-defining

whom

people in formal styles or in writing; often with a preposition; rarely in conversation; used instead of who if who is the object

defining and non-defining

no relative pronoun

when the relative pronoun defines the object of the clause

defining only