Back in the day, I owned a Palm Pilot and recall the touch screen behaving far differently than iOS devices in that pressure was required (and felt) to signal the processor. This type of digitizer is called a “resistive screen,” and takes advantage of finger or stylus pressure causing two thin layers to come into contact at a precise point, thus changing the resistance and signaling an instruction to the processor. These screens were predominant back when Palm Pilots were all the rage — I can assure you they no longer are.
We know that iPhones and iPod Touches are the rage though and they use what are called “capacitive touch screens,” relying on the conductivity of your skin to affect changes via signal path. The display houses an LCD, a glass substrate containing a very thin conductive layers, another glass substrate containing another very thin conductive layer, a transparent “bonding” layer, and finally the glass cover that you actually touch. On later models there is yet still another fine anti-reflective, oleophobic — from the Latin “oleum” (oil) and Greek “phobos” (fear) — layer.
So if you were to get up close and personal with the screen — using a microscope of course — you would see a grid array in the glass substrates: the bottom layer houses very thin, vertical, metal strips called sensing lines that detect electrical current at intersections (“nodes”) with the horizontal lines of the top layer, called driving lines. The driving lines carry current whenever the screen is on so that any interaction with the fingertip or stylus can be registered. This symbiotic relationship of the two layers is called mutual capacitance.
Because living creatures have capacitance (we can store / conduct electricity), when you touch the screen, you are lowering the amount of charge at specific junctures as a small amount of the charge from the driving lines (why its substrate is closer to you than the sensing lines), transfers to your finger. Electrical impulses are sent to the processor for further analysis once the raw data of your touch is collected based off of its coordinates on the grid and its affect on the circuitry.
The processor uses several important pieces of software, built into the device’s memory, to analyze your electrical impulses. It needs to know the size, shape, and duration of your touch; movement (for gestures), placement, context (what application are you trying to use), and it needs to know whether there are one or multiple simultaneous touches. Apple has engineered the software to recognize a relatively limited set of possibilities so that if the software receives raw data that is unknown, it will not acknowledge the signal as a touch.
As with all modern day computations, the aforementioned sequence of events happens in nanoseconds, giving you the impression that you’re executing commands in real time with your finger. If you feel like your screen is not responding the way it should across all applications, there is likely an electrical fault that requires a replacement.
These incidents, however, seem few and far between due to technological advancement (manufacturing techniques) and that the LCD/digitizer/glass is all crammed into one thin component. You may notice that cracked glass rarely renders the digitizer useless because you’ve only damaged one of the several layers comprising the display. You should seek to get it fixed regardless so that there is no breach in the system and so you don’t hurt yourself!