The modern notion of a bigot extends beyond mere prejudice. Everyone has prejudices and prejudgements about other groups, because it's easier to have a prejudice than to spend time getting to know an individual. This is a normal and expectable, if not necessarily excusable, way for the human brain to conserve processing time.
Bigotry goes above and beyond this, however. A bigot takes his prejudices and adheres to them, stubbornly and without apology. He maintains that his prejudices are true and right, without giving any consideration to the alternative. The difference between mere prejudice and outright bigotry is the difference between unconscious thought habits and conscious decisions of belief.
"Bigot" is usually synonymous with "racist" these days, but it's not restricted to any one form of prejudice. I've known one who describes all homosexuals as "faggots", and another who regards all Christians as "creationist dupes". I've known an Irishman who thinks of all Americans as self-centered and liberitarian. And, in more recent weeks, I've encountered a zealous anti-Communist who perceives anyone with a liberal slant as being part of a vast pinko conspiracy.
It doesn't have to be anything as big as racism or "gay bashing" or misogyny. It comes down to a person's willful choice to believe anyone who falls into a particular group as possessing certain traits, and an obstinate refusal to believe otherwise. Bigotry is prejudice to the power of egotism.