Kingdom Hearts is action adventure series by Tetsuya Nomura, that has cameos from several popular Final Fantasy characters, mainly from Final Fantasy VII, VIII, and X. The game mainly centers around a teen named Sora who loses his friends Kairi and Riku when their world is destroyed. Donald Duck and Goofy help him look for his friends across various Disney worlds. Sora's weapon is a keyblade, a mix between a sword and a key, and it is used to lock the sleeping hearts of the world, which is a corny plot.
Some fans are a little too overzealous and end up looking really bad when they end up comparing Final Fantasy to Kingdom Hearts on a FF board.