The image and sound on your TV are captured by a videocamera and coded into data that is stored on a computer (this used to happen on a piece of magnetic tape, but the idea is the same). The data can be coded in series of 1's & 0's -- a lot of 1's & 0's actually.

Imagine taking a picture then drawing boxes on it that are very small. For each color you can assign a set of 1's & 0's -- yellow could be 0001, blue could be 0010 and green could be 0011. You can break up the picture as finely as you'd like and then write a very long series of 1's & 0's to represent it. Now all you'd need is a way to send it to someone and they could decode it into a picture if they knew your code.

For wireless -- over-the-air -- transmission, the data is sent using a coded electromagnetic wave from the transmitter to an antenna on your TV. What's an electromagnetic wave? You already know one form -- visible light. The different colors of light are different frequencies of electromagnetic waves. The waves used to transmit TV and radio are invisible to your eye so you don't see them (just like microwaves!)

The encoding is a bit complicated, but you can imagine it as if you have a light that you turn on and off very quickly to send a message. For a one, you could leave the light on for 10 seconds and for a zero, you could leave it on for 2 seconds. To transmit TV signals, a large tower creates a signal that essentially sends 1's & 0's extremely quickly by switching an electromagnetic wave that then travels as invisible light.

The antenna on your TV or radio can pick up that wave and decode it into sound and a picture.

The same idea works using wires, but in that case the data is carried by changes in electrical voltages on an wire.