With a VGA signal, a pulse on the vsync line tells the monitor to start at the top left. A pulse on the hsync line tells the monitor to move down a row. In between those pulses, the analog values of the RGB values determine the color as the beam scans left to right. The CRT is dumb -- it's up to the device sending the signal to time the pixel values correctly so they hit the screen as the beam scans over the correct spot on the screen. If you were making a device that needed to display analog video, you'd have to make sure the signal you were outputting would make the electron gun fire at the right time. (e.g.: http://www.tinyvga.com/vga-timing)
> How does it move so fast?
They're electromagnets, so they move in sync with the timed signal sent to them.