Many mortals (including the English, of course) are confused by these words.
Many moons ago (hace muchas lunas) the definitions or meanings of these words were the opposite to the meaning today.
Here are the meanings in use today.
First, distinerested. If you are disinterested (imparcial, or “impartial” in English) you are not biased. Perhaps you have an open mind, without prejudices (prejuicios).
Now, uninterested. If I am uninterested (no interesado) in something or someone, then I am not interested in the subject or person.