Geophysics is the study of the Earth using physics to determine Earth properties. Some of the physical properties studied are the earth's size and gravity, the weather and atmosphere, volcanoes and hot springs, the oceans, water in the ground, and the movements through the earth (otherwise known as seismology).
Seismology studies seismic waves that move through and around the earth, including earthquakes. Geophysicists can study how these waves move and affect the earth to learn the nature of the earth deep below the surface, where it can't be directly seen. The seismic waves can be from a naturally occurring event, like an earthquake, or they can be artificially produced.