Altimeters
The easiest way to develop an altimeter is by measuring directly the atmospheric pressure (see 
AN501 "Using MS5534 for altimeters and barometers"). Atmospheric pressure represents in fact the weight of the air above the pressure sensor.
Since the amount of air present above the sensor decreases when the altitude increases, we can modelize the relationship between pressure and altitude.
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Pressure sensors are used in a wide range of medical applications. Most popular devices are electronic blood pressure meters that use precision pressure sensors to measure the cuff pressure (0-300 mmHg) and detect the pulse. Another possible application is the sport watch displayed on your left which uses the pressure indication to determine the effort of runners (detecting, for example, if the runner goes up or down hill).
The primary application of pressure sensor in aerospace applications is the measurement of airplanes' altitude. But pressure sensors can also be used for velocity measurement, like in Pitot tubes for example. Variometers, measuring vertical speed also use pressure rate of change to estimate climbing or falling. But pressure sensors might also be used in more complex systems, like in wind-tunnels when designing wing profiles.