Accelerometers can be described as two transducers -- the primary transducer, typically a single-degree-of-freedom vibrating mass which converts the acceleration into a displacement, and a secondary transducer which converts the displacement of the seismic mass into an electric signal.
Most accelerometers use a piezoelectric element as a secondary transducer. Piezoelectric devices, when subjected to a strain, output a voltage proportional to the strain. Note that piezoelectric elements cannot provide a signal under static (e.g., constant acceleration) conditions.
Important characteristics of accelerometers include range of acceleration, frequency response, transverse sensitivity (i.e. sensitivity to motion in the non-active direction), mounting errors, temperature and acoustic noise sensitivity, and mass. High-frequency-response accelerometers are used mostly for vibration and shock testing. Shock testing includes automobile and aircraft crash testing, and studying the effects of explosions or earthquakes on buildings and other large structures.