Here is how the resistive touch screen looks like. A transparent screen having four wires coming out of it for its interfacing. Though transparent, but consists of 2 resistive layers lying parallel to each other one above the other having an air gap (a kind of insulation) in between. What happens is, that when the user touches the touch screen, the two resistive layers gets connected and a voltage divider resistive network is established. Here is the pictorial view of how the resistive network is established.

Now, we need to know the location, where the user have interrupted the touch screen. In order to know that, we will have to calculate the co-ordinates. To calculate the coordinates on the touch screen where the user has touched, we play with four wires coming out of the touch screen. The two wires indicating the x-axis and the other two indicating the y axis. (x and y axis here are the relative directions. So, after the formation of a potential divider network,  we apply voltages to X+ and X- and making Y+ OR Y- as a probe (means we take voltages from Y+ or Y-), in the same fashion we take voltage from the X+ or X-. These two voltages we get are function of the resistance which will vary through out as you move in the touch screen.

Now, the two analog voltages you get are fed to the adc for its digitization. What is left now is the mapping of the touch screen as per the requirements. How to interface resistive touch screen with the microcontroller using resistive touch screen controller will be discussed later.

More to follow!