In the past decade or so, it's hard to imagine a world without touch-sensing electronics. Smartphones are a prominent and ubiquitous example, but of course, there are many devices and systems that integrate touch-sensitive functionality. Both resistance and capacitance can be used as means to achieve touch sensitivity.
While applications based on capacitive touch sensing can be quite complex, the fundamentals of the technology are fairly simple. In fact, if you understand the nature of capacitance and what determines the capacitance of a particular capacitor, then you have a good understanding of capacitive touch sensing.
Capacitive touch sensors fall into two broad categories: mutual-capacitance configurations and self-capacitance configurations. The former, where the sensing capacitor consists of two terminals that act as transmit and receive electrodes, is preferred for touch-sensitive displays. The latter is one terminal of the sensing capacitor to ground and is a simple method for touch buttons, sliders or wheels.
So what causes these changes in capacitance that the touch-sensing controller is going to detect? Of course human fingers.
Why does the presence of a finger change the capacitance?
There are two reasons: the first involves the dielectric properties of the finger, and the second involves the conductive properties of the finger.
We generally think of capacitors as having a fixed value determined by the area of the two conducting plates, the distance between the plates, and the dielectric constant of the material between the plates. We certainly can't change the physical size of a capacitor just by touching it, but we can change the dielectric constant because a human finger has different dielectric properties than the material it replaces (probably air). It's true that the fingers won't be in the actual dielectric area, which means directly in the insulating space between the conductors, but this "intrusion" into the capacitor itself is unnecessary.
It turns out that human flesh is a good dielectric material because our bodies are mostly water. The dielectric constant of a vacuum is defined as 1, the dielectric constant of air is slightly higher (about 1.0006 at sea level and room temperature). The dielectric constant of water is much higher, around 80. Therefore, the interaction of the finger with the electric field of the capacitor represents an increase in the permittivity and therefore an increase in the capacitance.
Anyone who has ever experienced an electric shock knows that human skin conducts electricity. I mentioned above that direct conduction between the finger and the touch-sensitive button - which is what happens when the finger discharges the PCB capacitors - doesn't happen. However, that doesn't mean the conductivity of the fingers doesn't matter. It's actually very important because the finger becomes the second conductive plate for the additional capacitor.
For practical purposes, we can assume that this new finger-generated capacitor (which we call the finger cap) runs in parallel with the existing PCB capacitors. The situation is a bit more complicated because the person using the touch sensitive device is not electrically connected to the ground node of the PCB, so the two capacitors are not "parallel" in the typical circuit analysis sense.
However, we can think that the human body provides a virtual ground because of its relatively large capacity to absorb electrical charges. In any case, we don't need to worry about the exact electrical relationship between the finger cover and the PCB cover; the important point is that the pseudo-parallel configuration of the two capacitors means that the finger will add to the total capacitance as the capacitors are added in parallel.
Therefore, we can see that both mechanisms that control the interaction between the finger and the capacitive touch sensor contribute to the increase in capacitance.