Analog video basics
Analog video is the use of analog signals such as voltages to encode video signals. Video signals are often thought of as sequences of still image "frames", but an analog video signal is physically just a continuously-varying waveform and can be regarded this way as well. Conversely, any analog signal can be treated as an analog video signal by feeding it into analog video equipment, though signals sufficiently different from the input the equipment was designed for may not produce any visible image. Some of the most widespread technology for creating, working with, and displaying analog video was developed for analog television, and variations on the signal formats used for analog TV broadcasts are commonly what people mean by "analog video". The signal most similar to over-the-air analog TV broadcasts that is worked with most commonly is composite video -- this is a color analog TV signal that has not been RF modulated for radio broadcast.
Cathode ray tubes can be used to build analog displays by scanning an electron beam across a piece of glass (the screen) that has been chemically treated so that it lights up when electrons hit it. The analog video signal is used to control how fast the electron gun is emitting electrons, that is, the current flowing through the cathode ray tube, which can be controlled with electronics. To display a continuously-varying analog video signal as a sequence of images, it is necessary to map a one-dimensional signal, such as voltage over time, into a two-dimensional signal, such as brightness of each point of the screen. This process of splitting up an analog signal over time so that it forms a sequence of horizontal lines on a display that are perceived as an image is called scanning, and each horizontal line is called a scanline. The position of the scanlines, where they begin and end, and where the first and last scanlines fall is determined by synchronization pulses.
Many devices produce or process analog video, such as cameras, VCRs, video game consoles, Raspberry Pis, video synthesizers, computer graphics cards, and TV studio equipment such as video mixers. The analog nature of these signals allows for unique ways of handling and combining them, for example deliberately glitching a video feed with a circuit that changes the signal, circuit bending existing devices to achieve new behavior, or plugging analog sources such as audio signals into equipment expecting analog video.
Initial set of links from this scanlines post