"CRT" is an acronym for cathode ray tube. It is an evacuated tube used primarily for monitors, although it is largely obsolete as of 2014. Most newer monitors use light emitting diodes or a liquid crystal display.
CRT monitors have a general boxlike shape and cannot be thin flat screens like the modern LED and LCD. This is because of the funnel-shaped vacuum tube that channels the cathode ray to the phosphor screen seen by the viewer. The tube is encased inside coiled wires that create a magnetic field, which acts on the electrons inside and directs them to specific points on the screen to form the images on the monitor.