The relative pronouns are who, whom, and whose; that and which; and where.
--Use who and whose in reference to persons.
He's the clown who preached for 35 minutes.
The person to whom I complain about long sermons is my 6-yr-old.
DPs are the guys whose sermons are always too long.
NOT: The pastor that preached forever also talked about himself the whole time.
--Use that and which in reference to objects or animals.
The Athanasian is the creed that no mother of a 1-yr-old ever confesses in its entirety.
The gun on the lintel is for untethered dogs which enter my yard.
--Use where in reference to places.
That's the rest stop where we had to get the world's nastiest diaper out of the car.