I was under the impression they were the same thing, but I have a friend who insists they're two different trees. What's the difference, and what's the relationship between the two?
They are the largest and tallest trees in the world. They are related but not the exactly the same. See Sequoioideae.
As far as growing regions go:
Redwood trees primarily grow in Northern California and Oregon along the coast. See Redwood National Park.
Sequoia trees primarily grow in Central California in the Sierra Nevada mountains. See Sequoia National Park.