if you have played KH1, Kingdom Hearts :Chain of Memories takes place between the two.
Kingdom Hearts: CoH takes place immediately after Kingdom Hearts, I'll just say the main idea here as to avoid spoilers for you in case you want to play it. Basically the game introduces the Organization XIII a little more in depth, upon entering the castle he is warned that the further he goes the more loss in memory he will incur. He travels to the very top floor forgetting many things on the way, after defeating the final boss, a girl named Namine ends up with the power to restore your memories, while getting your memories restored you must be placed in a hibernation state. That is where KHII starts.
Once again a VERY basic story line. If you want to go in a little more depth I would have figured you'd end up looking up the info.
