Your computer's keyboard, mouse, and other input devices are the primary channels that you can use to send commands and data to the CPU. Both the keyboard and mouse contain small, self-contained computers that convert electrical impulses to data and transmit that data to a buffer inside the computer.
Because you use the keyboard and the mouse so often, it's extremely important to find models that fit comfortably under your hands. Using the wrong ones for your particular needs can literally become a very painful experience.
Other input devices are also available, and they might be better choices for some users. These include touchpads, graphics tablets, and even bar code readers. But for most of us, the mouse and the keyboard will be the most common physical interface between you and your computer.