Touch - History
The idea for touch screen technology has been around for a long time. There were theories as early as 1965 with patiends filed in 1972. The first multi-touch system, a system that can track more then one point of contract, was made in 1982 using a camara and a sheet of class. Obviously very cumbersom and slow to operate.
Touch really took off in 1993 with the first phone and became a comerical success in 2007 with the first IPhone released by MAC.
Touch - Hardware
There's many types of touch interface system but only two have been used on the commerical market.
Resistive systems are like keyboards. You litterally press down on the screen which puts pressure on a sensor. Resistive systems are cheap and durable but not very persise. Their use has been restricted to things like ATMs and vending machines.
Capacitive systems detect the electiricty, when properly calabrated, of a person's finger. These systems are eaiser to use multi-touch technology with and are far more percise making them the industry standard.
Both systems effecticly create a grid and detect how the user's figer moves through it to determine what happens. New advances allow the screen to determine how hard the user is pressing and how many figures allowing for quick use of shortcuts built into the system.