If you hook up your Arduino via the USB-port, due to the USB converter on the Ardiuno it'll appear as a serial device. For keypads, mice and joysticks, the Device should identify itself as class HID (Human Interface Device). There are some solutions to do that in software, but it's not as good as those processors which offer a USB-stack in hardware.
If you look for "Arduino USB HID" you'll find plenty of information on the matter.
Korman