For instance, the following is an opinion. "I don't like girls." It is a subjective statement about your personal feelings.
The following is not an opinion: "Girls are dumber than boys." It is an objective statement about reality. It doesn't explicitly say how you feel about girls; it makes a testable claim about the real world, and thus is subject to scrutiny, and ultimately, a ruling on its accuracy.
In conclusion: An opinion is a statement about how you feel about something. If your statement ends up saying something about anything other than you, it is not an opinion, and can be proved to be wrong.
Addendum: Another thing. Even when "it's my opinion" is used on something that is an opinion, it still doesn't mean that the opinion can't be examined! Though something subjective can't be "wrong", per se, it can still be misguided. Saying "it's my opinion" doesn't mean that everyone should just accept your feelings on something and not examine where they come from or why you feel that way.
Addendum: Another thing. Even when "it's my opinion" is used on something that is an opinion, it still doesn't mean that the opinion can't be examined! Though something subjective can't be "wrong", per se, it can still be misguided. Saying "it's my opinion" doesn't mean that everyone should just accept your feelings on something and not examine where they come from or why you feel that way.