Typically I just ignore these Writer's Block things, but this one's actually an interesting question, so I'm going to run with it.
The Compact Oxford English Dictionary defines a hero as follows:
The gender specificity in that definition comes, of course, from the fact that female heroes have their own word: Heroine, which is defined in exactly the same manner as Hero, except that the genders are swapped and the third definition is missing. For the purposes of this writing, we'll consider the word 'hero' to encompass both the male and female varieties, because damn if accounting for both words wouldn't be a pain in the neck.
It doesn't take much to satisfy the last two definitions, and neither of them are applicable to real life, so clearly the real question is what makes someone fit under that first definition?
Well, whatever you do to deserve the title of 'Hero' in terms of courage, it has to be difficult. If you aren't overcoming adversity, what you're doing isn't heroic, it's just...nice. At best.
What does that mean? It means that a multi-billion dollar corporation donating ten thousand dollars to charity is nice, but not significant, since they spend more than that on paper clips every year. But two guys in their basement organizing a million dollar charity? That's something else.
Watch almost any movie that has 'based on a true story' on the poster, and you can see an example of heroism based on courage. The protagonist has a goal, but also has a handicap: maybe he or she is too old. Or they have bad legs. Or they're poor, and nobody will listen to them. But they keep working at it, keep trying, until eventually they overcome whatever was holding them back and claim that victory.
That sort of courage and determination resonates with us because we all have goals and dreams, and sometimes these can seem impossible to us. The odds seem too long, the chances too slim. But these stories empower us with the knowledge that sometimes, people do succeed, and if you work at it that person could be you. They inspire us to pursue our dreams, and ignore the odds; as Simon said, 'As long at there's even a one percent chance, that's as good as a hundred!'
A heroic achievement, on the other hand, is when you give of youself to help people who need it. You give your time, your money, your effort, to take a downtrodden person or people, and lift them up a little. This ranges from the extraordinary (distributing food to an impoverished region, diving into icy water to save a drowning child) to the utterly mundane (one child protecting another from bullies). To put it in more dramatic terms, the person felt completely alone, downtrodden, belittled, with nobody who cared enough to help...and then suddenly there you are, standing in front of them, helping them overcome whatever ails them and reminding them that hey. You're not alone.
People see these sort of heroic actions and they want to emulate them. They want to be the ones who make the world a better place.
So, what makes a hero? I could go on and on forever describing it, it's a very complex concept, but when you come down to it it can be described inv ery simple terms:
A hero is someone who makes the world a better place. And then inspires other people to do the same.