An accelerograph can be referred to as a strong motion seismograph, or simply as an earthquake accelerometer. They are usually constructed as a self-contained box, more commonly now being connected directly to the Internet.
Accelerographs are useful for when the earthquake ground motion is so strong that it causes the more sensitive seismometers to go off-scale. There is an entire science of strong ground motion, that is dedicated to placing accelerographs in the vicinity of major faults. The type of information gathered (such as rupture velocity) would not be possible with the standard seismometers. The best known example is the Parkfield Experiment which involved a massive set of strong motion instrumentation.
Within the accelerograph, there is an arrangement of 3 accelerometer sensing heads. These are usually micro-machined (MEMS) chips that are sensitive to one direction. Thus constructed, the accelerometer can measure full motion of the device in three dimensions.
Unlike the continually recording seismometer, accelerometers nearly always work in a triggered mode. That means a level of acceleration must be set which starts the recording process. This makes maintenance much more difficult without a direct Internet connection (or some other means of communication). Many trips have been made to accelerometers after a large earthquake, only to find that the memory was filled with extraneous noise, or the instrument was malfunctioning.
Accelerometers are used to monitor structures for earthquake response. Sometimes, with the data, a response spectrum is computed. Other analysis is used to improve building design, or to help locate important structures in safer areas.