When I take a photo of a screen such as a laptop or television, what are the vertical lines that appear in the picture?

When I take a photo of a screen such as a laptop or television, what are the vertical lines that appear in the picture?

Screens do not display images continuously. They display dozens of images per second, one right after the other. Your brain interprets this as a continuous image, but it isn't really. For instance, my monitor is functioning at a rate of 60Hz right now, i.e., it's displaying 60 images per second. This is not to be confused with frame-rate in game rendering, for instance, which is a measure of how many frames your computer can process in a second. Your monitor will always display the same number of FPS, and if the data coming out of your computer is less than that, some of those images just get displayed more than once. This is when even a game which is really lagging will still appear continuous on the monitor: the monitor is still happily churning out its 60Hz, even if it's only getting 15FPS from the video source.

Further, the image is not displayed all at once. Old CRT monitors had an electron gun which would sweep the screen from one side to the other (frequently top to bottom), displaying the image that way. This is called progressive scanning. LCDs don't have electron guns, but they use progressive scanning too. In essence, each line of the monitor is activated 60 times per second, but only one at a time.

Again, the eye interprets anything above about 30Hz as a continuous image. But cameras frequently have a shutter speed--i.e., the time that the camera permits light to reach it sensor in taking a single image--of far, far more than that. Even consumer-grade digital cameras use a shutter speed which approximates a frequency of 250-500Hz in many cases. This is fast enough to catch the monitor displaying a partial image. Hence the visual artifacts in the resulting photograph.