Seismology is the science of studying earthquakes. Seismologists are scientists who practice seismology (some of them get good at it, too ;-) ). We record ground shaking with an instrument called a seismometer, and the instrument makes a recording on a device called a seismograph -- sometimes on paper with ink, but mostly these days with digital computers. The recording itself is called a seismogram.
Most classical seismometers have either a heavy mass on a suspension system, like a spring, or a mass at the end of an arm which swings like a fence gate. Seismometers work by sensing the relative motion of the heavy mass and the frame of the seismometer itself. The mass ``wants'' to stay in one place due to inertia, while the frame of the instrument has to move with the ground, since the frame is firmly attached to the ground. This relative motion is sensed by the instrument, and is what is recorded by the seismograph to make a seismogram.
Charlie Thompson has built a homebrew seismometer which has the ``fence gate'' design.
Most modern instruments are actually completely computerized, and work by sensing how hard they have to work to make the mass move with the rest of the instrument. This record of the force necessary to make the mass move is stored digitally in a computer connected to the seismometer, and sent via phone lines or the Internet to a processing center, where seismologists use computers to look at the records and play with the earthquakes. These days, most seismic data processing never actually involves paper records -- though I have one of my favorite paper records in a frame on my wall.
The folks at the UC Berkeley Seismographic Station have a really cool site where you can make your own seismogram.
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ES 10 Lecture
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Seismic Waves
Greg Anderson