What makes the difference between the quality and clarity of different monitor screens?

So what does 'dot pitch' mean?"

Think of dot pitch as the size of the smallest physical visual element on the display. As far as the PC is concerned a pixel is the smallest programmable visual element and depending on what resolution you're in, a pixel can include one or more physical screen dots or elements.

Technically, in a cathode ray tube (CRT) display with a shadow mask, the dot pitch is the distance between the centres of the holes in the shadow mask, measured in millimeters.

The shadow mask is a metal screen with loads of holes punched in it, through which the three electron beams pass (Red, Green & Blue) that focus to a single point on the tube's phosphor surface thus displaying a pixel of the chosen colour. So, the smaller the dot pitch, the smaller the dots are that the physical display is made up of, and therefore the finer the overall image will be.