Digital pictures are made up of milions of small "dots" called pixels. If you zoom in close enough to a digital picture you can see the pixels.
Each pixel has 3 colours (red, green, blue) mixed together in different strengths to make the "main" colour of that individual pixel.
The "strength" of each colour can be a number betwen 0 and 255. So a pixel that is mostly red may be 240 for red, 120 for green, and 50 for blue and so on.
The camera has a "light sensitive" device at the back which will look at the light coming in through the lens and measure the strength of each colour (red, green, blue) for each pixel.
Then all these different pixels of the different colours, when lined up in rows, become the whole picture.
The information is stored on the magnetic card in digital format. Which neans it is stored as binary information consisting of millions of 0 (zeroes) and 1 (ones), such as 0100 1101 1011 1111 0011 1100 and so on for millions and millions of them.
This string of binary digits is "read" by the camera or computer which converts it into pixles and then as a digital image.
Music on CD and films on DVD are also stored in this binary format, so a song on a CD would also look like 0101 1100 0101 1011 0001 1111 and so on.
Freeview and Sky signals sent to your TV, and your voice on your mobile phone, are also sent through the air as binary information.
Digital media is good because it cant degrade (like old photograhs used to for example) because this string of zeroes and ones will always stay the same no matter what happens to them.