Opinion: a subjective statement or thought about an issue or topic, and is the result of emotion or interpretation of facts.
Belief: Mental acceptance of a claim as truth
Opinion is how you rate something on some sort of scale. From good to bad, large to small, ugly to attractive. Belief is what you feel to be truth, with no scale; it's your perception of reality. "She's ugly," is an opinion based on a personal preference, and is no more or less valid than anyone else's opinion. It cannot be proved or disproved because it is subjective. "She's dead," is a belief, which may or may not be true and can be either proved or disproved with evidence. A theory is a belief that cannot yet be proved or disproved. "The sun has tiny firemen in it," is a theory. If it were possible to look inside the sun, one could prove or disprove the tiny firemen theory. My personal opinion is that it is a stupid theory.
The reason I bring this up is that many people are out in the world spouting complete untruths and then declaring them to be opinions. "Gay is a choice, Obama is Kenyan, The Earth is flat." And then when you try to correct them they say "That's just my opinion," because their opinions are so sacrosanct that you are not allowed to argue them. And that would be true, if they were stating opinions, but they are actually stating beliefs, or at the very least theories, and generally false ones at that. Obama has presented a mountain of evidence to prove that he was born in Hawaii. Some people choose to believe he wasn't, but that believe isn't an opinion.
Also, on a related note, while I may not be able to disprove an actual opinion, I can still disagree with it and react accordingly. You are very much entitled to believe that homosexuality is icky and bad and morally dangerous, but I am equally entitled to hate you for it. Opinions may be sacrosanct, but they do not come without opposition.