Well, first off, today’s LCD computer screens won’t get messed up by magnets (unless they’re EXTREMELY STRONG). But CRT (cathode ray tube) monitors can get messed up by magnets. Those more boxy screens work by firing electrons extremely precisely at millions of tiny phosphorescent dots. Hit the green one…green light…the red one…red light…hit the blue one…blue light. Pretty remarkable that we figured out how to turn that system into 50 years of color TV.
So, as you probably have guessed by now…electrons (being negatively charged) are affected by magnets. If you plop a magnet onto a CRT screen, it will either attract or repel the electrons, distorting the image. It’s pretty cool to watch. Less cool, though, is that the film behind the class can actually get permanently magnetized by a magnet, resulting in the screen becoming permanently distorted. So don’t do that unless you don’t care about your monitor anymore.
Today’s LCD screens don’t have any flying beams of electrons for magnets to distort…but they’re still very cool.