A seismograph is an instrument installed in the ground that records and measures the vibrations produced by earthquakes.
If you throw a rock into a pond or lake, waves will ripple outward in all directions from the point where the rock hit the water. The same thing happens during an earthquake: it creates seismic waves of vibrations in the ground that radiate out in all directions. The further these waves are from source of the earthquake, the less energy they have, and the harder they are to detect. Scientists want to detect them, however, because they learn useful things about the Earth's many-layered interior from them.
The Earth's motion during an earthquake must be compared to an object that doesn't move when the ground shakes. That's why a seismograph uses an object -- usually a large electromagnet -- suspended on springs within a case made of fine wire, to keep the object still, while the case around it moves with the ground's motion. (This is similar to the way shock absorbers work in cars.) As the case moves, it produces small electrical signals in the wire coil. These signals are then amplified and stored digitally on a computer, or transcribed onto a recording drum.
But a single seismograph pendulum only detects waves moving in one direction. To get a complete picture of the wave's motions from other directions, modern seismograph stations have three separate instruments to record the different kinds of waves: waves moving north-south and east-west, and the way the ground moves up and down.
The Global Seismograph Network goes one step further, linking 128 seismograph stations in more than 80 countries on all continents to collect and store seismic data from around the world that can be accessed by everyone.