The three answers already given exhibit deep thought and philosophical reasoning towards the better understanding of what makes an opinion and opinion, and what makes a fact, a fact. I am in awe by how thoroughly this question has been answered, and how all three questions have hit the mark so well that their opinions don't really collide with each other.
However, my opinion differs from theirs. A word to the wise, my opinion stems from common sense and colloquial reasoning, as opposed to the ways of thinking used in the other answers.
An opinion, to me, is a thought, much like a fact or an idea. I hope there's no disagreement so far, because from here on the field gets tricky, as the difference between thoughts lies completely inside of one's mind, as much as we have tried to standardize it through philosophy and psychology.
Inside of my head (and rooted deep inside the communication system(s) i partake in, some would argue), an idea is a thought applied towards an external thing. For example, "i wonder what the world would look like with pink glasses on my eyes" can be an idea, given that i'm using a thought to [verb] about something out there (the world, in this case)
Likewise, a fact is a thought whose reasoning isn't completely clear to me, and which i accept because something or someone else has told me that it is true, so i choose to believe it/them. For example, if a book tells me that "most plants look green because of clorophyll", why, i'm pretty sure that'd be a fact.
An opinion, finally, is a thought whose reasoning is clear (not necessarily completely clear, but clear nonetheless) to me, and which i can update or fact-ify if so desired. (Maybe there's a better term for it, but what i call "to factificate" is to hold as a fact, that is, solid and unchangeable until forced to do so). For example, "i really disliked the singer's voice on last night's event" is a pretty solid opinion, given that it's very hard to change likes or dislikes unless doing so from the inside (like realizing your own voice isn't much different than the singer's)
Observant minds (or those whose opinions differ from mine and find said difference unpleasant), might realize that facts that come from other person's minds can also be opinions, and to take them as fact would be to misunderstand their nature. Indeed, somebody can tell me that 2+2 =5, and i'd have no reason why not to take it as fact. But here's where everything gets fun. I, as in the me who's writing hypothetical scenarios at 3 AM from my computer, believe 2+2=5 can be a fact as valid as any other. I mean, sure, pure maths and theory of numbers (and what we learned on elementary) tells us that the statement is not right, but that just makes it a wrong fact, not a "not-fact".
Wait, what?
"A "fact" must be true." - Fred BC
"Description of reality seems like a good definition of "fact,"" -noah364
"A fact is what we call a claim that has been verified as accurate to a high degree of probability." - Aahmused
Oh great, now i've gotten myself into a mess. Everybody else agrees on something i don't, how am i going to get out of this?
First of all, do "wrong facts" exist? I know that wrong "facts" exist, or at least google tells me so, with several "You won't believe these 30 "facts" aren't true" or "10 "facts" everybody knows are right (which are totally wrong)" lists easily available online. But there's always a way to avoid calling them fact facts. Instead, they are fake facts, or not-really-facts, or seemingly-facts-but-actually-something-else. So this doesn't really helps my case.
What about disproven facts? A while ago, earth being flat was a fact. Slightly less time ago, the moon being unreachable was a fact. Yet even less time ago, N-rays' existence was a fact. So here we have found some facts that are no longer true. Does this mean they are no longer facts? Maybe. But then what are they? Surely earth being flat isn't an opinion, given that it had tons of scientific backup. What about an idea? N-rays were a pretty cool idea, but their existence, not so much. That is, the existence of something isn't a thought applied towards an external thing, given that it already IS an external thing. You can't really call "existent" something just because you saw it in your dreams. Likewise, you can't prove something doesn't exists because it's not inside of your mind.
So here's the money shot. This is the definition upon which i'm gambling my entire opinion around: A fact is a thought from the outside, but, much like opinions, it can be updated.
So, having said this, the only difference between opinions and facts is that opinions need less deep thought than a fact to be so. In fact, facts need more than thought to be so! They need hard-gathered data from that scary thing outside your mind, the world, with which to compare that thing which you want to make facts about, whereas opinions can be formed instantly, by the very act of reasoning, and which i can use to navigate my social everyday conundrums with various degrees of success, depending on how well my opinion fits with other people's opinions.
And that's it.
Extra query: Are opinions good or bad, objectively?
NO.
Opinions aren't objective, and as such, they can't be judged objectively.
In fact, we can make a rule-of-sorts around this. The more objective an opinion tries to be, the less opinion it is, the more fact-i-fied it becomes. As such, maybe a good opinion is that which you hold as an opinion, and update accordingly regarding subjective criteria (likes, feels, other opinions), and a bad opinion is that which is too heavily influenced by external things (like facts, or other people) and/or is taken as a fact.
As usual, the disclaimer goes something like "this whole wall of text is an opinion", but to be honest, it's a pretty bad one at that, following my own parameters stated a few sentences above, given that it's trying to cater to others, or at least, make other opinions look more like my own.
Huh, who would have guessed. Oh well, we can't always be winners.
Thanks for reading!