Yes, a computer mouse goes into the back of the tower, usually in the green lil hole. However, they do make usb mice (which fit into the small black rectangular slots) as well as wireless mice....
input devices are devices from which you enter data into the computer. Keyboard, mouse, joystick, tracker ball, graphics pad, scanner, video digitizer, mark sense reader, OCR reader, light pen, laser...
Yes. Any mouse is an input device.
To make it simpler: Any device that SENDS data or instructions/commands to the computer can be considered as an input device.