In my eyes its a platformer with RPG elements with a story that copy and pastes emo final fantasy characters with Disney Characters. It has a complex story that makes no sense other than you play the role of a boy named Sora who gets sucked into a portal and has to kill some dudes. It has a girl and a guy as protagonists. Everyone cares about it but they never play any other games.
Take one part Final Fantasy characters, mix in two part Disney themed levels, and gradually add in a cup of yaoi fan service with each squeal in the series. Mix thoroughly until all semblance of a cohesive plot is blended beyond obscurity. And for added flavor, add a dash of a shitty Gameboy Advance card battle game which the audience is required to play though completely to understand anything that is going on when they play Kingdom Hearts 2.
A fairly straightforward series gone awry with time. If you're looking to get into the series, just start with the first three - KH1, Chain of Memories, KH2, since that's the most straightforward storyline, and also the more interesting games to play. Basically, they're action RPG's that feature Disney and Final Fantasy characters, but the series itself follows its own storyline.
Kingdom Hearts is an Action RPG(I think...) series developed by Square Enix(or SquareSoft or whatever they call themselves now). The series is actually a collaboration between SquareEnix and Disney, so it features characters from both companies, such as Cloud and Squall from Square Enix's famous Final Fantasy series and Disney's Donald Duck and Goofy. However, the main characters are original and do not originate from previous games.
The main characters are different in each game, but basicaly the whole plot is that some bad guys wanna conquer all the worlds and you gotta stop 'em... The plot is more or less different in each game, but that's pretty much the whole idea...
The game plays like an RPG. You fight enemies to gain Experience Points(or EXP or XP for short). When you earn enough EXP you level up your stats, such as Attack(ATK) and Defense(DEF). So, the higher your level, the better. The characters also have abilities, but they need Ability Points(AP) to equip them. The more APs you have, the more abilities you can equip. Some abilities activate automaticaly(such as High Jump), while others need to be executed(such as Guard).
Instead of using turn-based battles like most RPGs, though, the Kingdom Hearts series features real-time battles and has a mini-battle menu. The battle menu is your way to victory! It has four commands: ATTACK, ITEM, and SUMMON are the main commands. *The 4th command varies depending on the situation. * You gain Summons as you play the game. Different summon characters have different abilities. Some fight with you while others heal you.
MAIN CONTROLS: You control your character with the left analog stick. You can use the the D-Pad(or the left analog stick in the first game) select a command in the mini battle menu. To select/confirm anything in the game or execute a command simply press the X button. The O Button makes the characters jump and cancels options. You can perform certain actions with the Square button. The Triangle Button usually opens extra menus.
SECONDARY CONTROLS: To adjust the camera use the R2 and L2 buttons in the first game or the right analog stick in the second one. Use the R1 button to lock on enemies. This is very useful, make sure to use it!
Sorry, I haven't played the PSP game in a long time and I never played the DS game(s), so I dunno the controls for the games on those consoles.
Personally, I love the series and if you're a fan or Disney or Square or RPGs then I highly recommend trying out this series, especially the PS2 games!
I hope I helped. Sorry for the super-long reply, but I wanted to include as much info as possible! ^_^'