4-Wire resistive touch screen is the preferred solution for low cost applications.
It consists of a conductive bottom layer (ITO Glass or Film) and a conductive top layer (ITO Film).
The two layers are suspended by many very small spacer dots. When a certain point is touched, the top layer contacts the bottom layer and a voltage charged by the controller is applied to the bottom layer's electrodes.
The coordinate X axis of the touch point can be detected by measuring the voltage value thru the top ITO layer and the coordinate Y axis of the touch point can be detected in the same way by changing voltage input to the top layer's electrodes.