What causes electro magnetic interference? like for example, the fan is on and the tv gets all weird

Anyways, the exact situation you are describing is caused by the magnetic field in the electrical motor of the fan inducing electiricty into the circuitry of the CRT in your television set. A similar phenomenon can be created by placing an electrical cable carrying 60/hz 120vAC next to a co-ax cable carrying a video and or audio signal which is somewhere around 5vDC. The magnetic field generated by the electricity running through the AC cable induces voltage into the video signal that causes what video engineers dreadingly refer to as 60 cycle hum. A different type of hum can also be caused by a ground loop from using two different electrical sources that are on different phases in a video system. In and audio system these exact same phenomenon cause what Audio engineer dredgingly refer to as a buzz. The exception is when they go into triple overtime trying to find out what's causing it.