whom - the dative case of "who", sometimes regarded as a formality
which - used to introduce a nonrestrictive clause if the pronoun refers to an inanimate object
that - used to introduce a restrictive clause and can refer only to inanimate objects in some speakers' idiolects
Pronounciation
hoo
hoose
hoom
which
that
Example Usage of Relative
milo9: @davidsirota When the Revolution comes, I'll be calibrating my Relative celebrity.
militrissa: Why do people gush over Proust? I'd rather visit a demented Relative | Germaine Greer | Culture | The Guardian http://bit.ly/Jr3YO
RomanticHome: I found every Relative from Phila , Oregon to Greece!! and my future home town and favorite place ever..well so far..California-San Diego