Caribou live in arctic North America and Greenland.  They are large, elk-like creatures with massive antlers.

Reindeer are smaller than caribou and live in arctic Europe and Asia.  They were domesticated in Eurasia 2000 years ago, and are still domesticated by people today, in Scandinavia and Russia.  These people use reindeer for nearly every need in their economy/society -- for food, clothing, and shelter.  Some reindeer are even kept as pets.  And yes, they really do pull sleighs!  Interestingly, they also happen to be very strong swimmers!
On Land, By Sea, and In the Air!
"Arctic Studies Center - Caribou & Reindeer."
Copyright Smithsonian Institution, 2002.
http://www.mnh.si.edu/arctic/html/caribou_reindeer.html