Well, to hell with it. This takes place before te kingdom hearts 1 & 2, so I will simply explain everything one would really need to know about the universe of kingdom hearts.
1. The Heartless
The heartless are beings of darkness wich exist because people have darkness in their hearts. When a heartless comes into being, it takes over the heart of the human it comes from, and the strength and appreance of a heartless is determined by exactly how powerful the heart of the person the dark consumed was. The weakest, most general class of heartless is called "Shadows", and they can actually turn into shadows, wich makes you unable to hit them until they turn into themselves again. If one has a really strong heart, he will basically turn into a powerfuller, more evil version of himself, usually keeping the same looks.
2. The Nobodies
When the darkness consumes ones heart, the heart, and therefore feelings, basically leaves the body. What is left, is a nobody. These usually appear as these odd, white beings, the weakest class being called "Dusks". If one has really strong willpower, he will appear sane and human as a nobody. However, he still won't have feelings anymore.
So, if for instance a person such as Xenaroth, a character from the series, who actually went trough this change, turns into a heartless, he will, because he has a very very strong heart, appear as himself as a heartless, but also have another one of himself without feelings, who also looks like himself.
3. The Keyblades and their wielders
The Keyblades are sort of like holy weapons. They are all shaped like keys, and given to the chosen ones. They aren't given to them by other humens, simply, they keyblades choose them. As they roam in worlds infected by the heartless (take note, once one turns into heartless, he can turn others into heartless) the keyblades can also be used to lock the hearts of different worlds wich will then protect them from heartless. The keyblades only serve those who they find to be worthy and will leave their wielders if they do not consider themselves worthy and whatnot.
4. The Kingdom Hearts
The Kingdom Hearts is the heart of all kingdoms. It will appear when one has collected enough hearts, and allows on the control the whole world, basically. Whenever one destroys a heartless, it releases its heart, wich can then be collected to create the Kingdom Hearts. The Kingdom Heart basically looks like a gigantic, heart shaped moon.
That's basically the things you need to know the most, I suppose. Basically, in the series, heartless take over people, keybladewielders bash them, some strong heartless, who is sane because he had a strong heart when he turned into one (other heartless work on instinct) collects them, turns them into kingdom hearts, tries to destroy the whole world, keybladewielders bash his head with their keyblades.
Oh yeah, and the process of a keyblade choosing its wielder. The wielder, still unaware of all this, is summoned into a realm with several platforms. On each of these platforms he is asked what he values the most, friendship and whatnot. This is in the games an event wich gives you and your partners some additional stat boosts, but basically, I consider this as a trial of is one worthy to wield the keyblade. If all guestions are answered and one is chosen to be worthy, he will face the darkside of his heart; this is usually very, very huge when it happens. They don't need to destroy it, for they can't, they simply have to face it face to face. It will try to consume one, but with the new keyblade, one must basically put his darkside on a leash and make sure it won't consume him, althought, let it be noted, sometimes one may need to face it again at some point. Let it also be noted that if one has a really strong heart, a really strong will, and is a keyblade wielder and the darkness consumes him, it is propable that the sane nobody and heartless born from this will also be able to wield keyblades. Or, the nobody can, at least - it was never shown if the heartless can, because the only keyblade wielder who turned into a heartless in the series turned into a shadow, the weakest class of heartless, wich works almost solely on instinct. Because the heartless are already consumed by their darkside, they propably do not need to go trough the trial and beat their darkside, but the nobodies must. However, the darksides of the nobodies have already left the nobodies - and hence, they face their lightsides. It is very much unknown what a lightside can do; my all my logic at least, it can't possibly consume you. But anyhow, you face it.