File this under the "well-known" items by long-time iPhone owners that we shouldn't have assumed all iPad owners would already know, as clearly not all iPad owners have iPhones.
If you've got a locked up iPad app or otherwise misbehaving iPad, and want to reboot / reset your iPad, simply hold the sleep and home buttons together until the iPad restarts.
You'll see the shutoff slider come up first, but just keep holding. The screen will go blank and the Apple logo will reappear as the iPad reboots.
Note: this does not reset any data on your iPad. This is just a restart, equivalent to rebooting your computer.