There are primarily two different types of touchscreen that dominate. Old established were resistive touchscreen. New thing is capacitive (or E-field) touch screen. (used on latest mobile phones, like iPhone). These have fancy controller for gestering interpretation, like finger swipes, tap, etc.
If you only need relative coarse (half a finger width) capability I have seen a touchscreen made with a row and column setup of IR LED emitters and IR detectors setup along the topside edges of LCD.