There are five relative pronouns: that, which, who, whom, and whose.
Who and whom refers to people.
Which refers to things, qualities, and ideas (never people).
That and whose refers to people, things, qualities, and ideas.
"In general, the relative pronouns that begin with the letters wh- are considered to be more literate. In contrast the pronoun that ... [has] a more colloquial flavor and [is] preferred in conversation."
(Douglas Biber, Susan Conrad, and Geoffrey Leech, Longman Student Grammar of Spoken and Written English. Pearson, 2002)
Click here for a link that provides some good examples showing the use of relative pronouns.
Common Core Standard