A fact is true. An opinion may NOT be true
A fact has real evidence and an opinion is just how someone feels on a relevant argument.
A fact is something that can be proven, while an opinion is something that can NOT be proven.

for example:
Fact: Martin Luther King was born on January 15, 1929 and was assassinated on April 4, 1968. (this is a fact because is can definitely be proven and is not based on what one person personally thinks)
Opinion: I think humans are more important than any other living creatures on earth. (this is an opinion because the whole idea is based on what you, as an individual, personally thinks or believes)
Hope that helped~