The picture on monitors with CRTs is produced by an electron beam which makes phosphor points glow. The intensity of the electron beam dictates how brightly the phosphor glows. Coloured pixels are produced by mixing the primary colours red, green and blue. Three separate beams of electrons are used to make the respective phosphors glow.
To make a picture out of various dots, the beam scans each point from left to right and from top to bottom controlled by magnetic fields. The necessary electromagnetic fields are produced in the so called 'deflection yoke'.