How does an airspeed indicator look from the inside?
The Indicated Airspeed (IAS) is measured based on the difference between the static air pressure (measured via the static port) and the ram air pressure (measured via the pitot tube). A typical GA airspeed indicator looks like this:


(source: Wikimedia)

The difference in the pressures causes the diaphragm to flex. This flexing is then mechanically translated into a rotation of the indicator. See this question for the full details: How does the mechanical linkage of the airspeed indicator convert dynamic pressure to KIAS?