Combination of camera and software. For camera, some use whatever built-in or peripheral web cam you have; others use a specially-designed camera (for which you will pay at least a few hundred dollars). Software tracks motion of head or some part of it, and translates that into mouse cursor movement. For mouse clicks, you either use the mouse, or a special keyboard key, or a switch, or software that clicks when you "dwell" the cursor for a certain amount of time.
Similar to head mice, but track movement of eyes, rather than head. Tend to be more expensive than head mice.
If you qualify for insurance to pay for a speech generating device (SGD), AND you can't use your hands, insurance might pay for one of these to go with it.
If you don't qualify for insurance to pay for one of these, consider other funding options on the Coping Financially page under the Grants section. I was able to get Team Gleason to pay for a PCEye Mini. These are also qualifying expenses under the Assistance Program of our local ALSA chapter.
Some wheelchair joystick controllers can be used like a wireless Bluetooth mouse, pairing with a mobile phone, tablet, or PC. The one I have can do this:
Software (requires a microphone) that translates spoken text into typed text or into commands to control your computer or smart phone, such as opening and closing apps, opening and closing windows, clicking on buttons and links, and controlling the mouse cursor.
An alternative to traditional on-screen keyboards. Has word-prediction built in. Free.