There are five (I think) methods used on touch screens:
1. Resistive (with different numbers of terminating wires)
2. Capacitive
3. Surface Acoustic Wave
4. Infrared
5. Strain Gauge
1 and 2 are the most common.
Resistve screens have two glass or plastic layers that are coated with resistive tracks. The two layers face each other and are seperated by small rasided dots. When pressed, the layers touch at that point and a current flows. This is measured and interpreted by the elctronics (usually just a single ic). Like here: