What's the difference?
This one has got an awful lot of people angry, particularly with those people who believe there is a ‘correct way’ of using language, and that these words do have two separate meanings, while there are those who believe that these two words can be used to mean the same thing as this is simply a development of the language.
So let me start with the meaning of uninterested, which most people are clear about. If you are uninterested in someone or something, then you don’t want to know anything about it: He was uninterested in the football match. The staff were totally uninterested; they seem to find working in a shop tedious and boring.
Now the official dictionary definition of disinterested is to not be involved in a situation, to have an opinion that is neither one side or the other: Some of that information has to be gathered by disinterested investigators, and not by politicians intent on proving things. However, many people use this word to have the same meaning as uninterested. The Collins Cobuild Advanced Learner’s Dictionary makes this observation: ‘Some users of English believe that it is not correct to use disinterested with this meaning [not interested]’.
Here’s the first example after typing in ‘disinterested’ in the British National Corpus: He follows the camp routines well, but at times he becomes disinterested and a little slow in carrying them out. My copy of Questions of English, published in 1995, also observes that the word ‘…seems to be used more and more often to mean ‘apathetic’ (a synonym for ‘not interested’)’, noting that this was, in fact, the original meaning of the word!