A CRT monitor uses an electroncannon to display whatever it shows. This electroncannon emits magnetic radiation, and it’s that radiation that causes headaches when you look at it too long, and is harder on the eyes.
An LCD using liquid crystals which are lit by letting a specific amount of electricity flow though it to color it red, blue and green. These are then combined to create any colour. Above the actual liquid crystals there is a thin film, on which the transistors are placed that manage these electrical flows. This film is the TFT film (thin film transistor). Behind the LCD layer is a backlight to, well, make the screen visible, heh.
Because there is no electroncannon, there is no magnetic radiation (which is also responsible for the distortion when for example you hold your cellphone next to your CRT monitor and get an incoming call) and therefore it doesn’t emit that radiation. It’s also that radiation that tires your eyes.
