|MadSci Network: Physics|
Most televisions still made today have a giant glass tube inside the box. It's similar to a light bulb in that it is a sealed container of glass containing specialized gases that assist in the display process. At the front of the tube is the screen. When you look at the TV screen, you're looking at the front of this tube.
At the back of the tube is a set of electron guns, a series of magnets and conductive plates that accelerate electrons through a high voltage difference. Televisions display an image by firing these electrons at the back of the screen, which is coated with a phosphorous material. When the electrons impact the phosphor, the phosphor is energized and it begins to glow. If the glowing phosphor is behind a red filter, the screen displays a red dot.
These electron beams are necessarily strong, to provide a bright clean picture, but they also represent a modest amount of electromagnetic radiation. While there isn't a lot of evidence that this radiation is harmful, it is one of the reasons your parents always tell you not to sit so close to the TV screen.
Since electrons are negatively charged, this bombardment of electrons against the back of this glass screen charges the back of the screen negatively. The glass acts like a bit of a capacitor, and so the outside of the screen becomes positively charged to balance the negative charge on the inside.
Some more sophisticated televisions have a grounding wire attached to the outside of the tube to dissipate this positive charge, but often, that's not the case.
The air is full of dust and particles, some are charged (or ionized), some are not. The particles in the air that happen to be negatively charged are attracted to the positively charged television screen. And so, the particles stick to the screen. Airborne particles can be anything: dirt, tiny flecks of dead skin, hair, fuzz, water, grease, pollen … just to name a few. Any of them can be negatively charged, and will be attracted to the static charge of the TV screen.
Once the TV is turned off, the charge dissipates, and in theory the particles should fall off. But they often don't, because all that dirt and water and grease and grime, etc. is now sticking to the screen because it's just sticky stuff. The static got them all together, and the natural stickiness of the things kept them together. This is why it's necessary to clean the screen periodically. This is also why you should avoid touching the screen with your fingers… oils from your skin will stick to the screen and make cleaning it more difficult.
As a side note, don't use drippy liquids to clean the screen. If the liquid drips down into the guts of the TV, something could short out and that would be the end of that TV. There are many cleaning products designed especially for TV screens, which clean and work to reduce the static build up of grime on the screen.
Thanks for your question!
Try the links in the MadSci Library for more information on Physics.