Believe it or not, these two words are not supposed to be synonymous. They have begun to overlap in usage, and there is some discussion as to whether it is even worth fighting it anymore. “Disinterested” has a specific legal meaning, so for my part, it is important to distinguish the two.
Uninterested generally means “indifferent,” e.g. “We did not invite you to the oil wrestling match because you seemed uninterested.”
Disinterested means “impartial” or “without bias,” e.g. “Since you don’t know any of the contestants, would you serve as a disinterested referee at the oil wrestling match?”
More and more, though, people say “disinterested” when they mean “uninterested.”
Photo credit: ‘Uninterested cat,’ photo by: Arinn, capped and submitted by: Andy, via icanhazcheezburger.com.