How do rock samples relate to Earth's Interior?
Rock is literally what the Earth is made of. The crust of Igneous, sedimentary and metamorphic rocks is just a tiny portion of all the rock in the Earth.
Different types of rocks, and subtypes of rock can reveal clues as to what is under the crust.
Igneous Rocks, for example, only occur where there's been volcanic activity, so the presence of these rocks can indicate where volcanoes are or have been. The presence of Granite in the Central USA indicates that there used to be volcanoes in that area, and the fact that it terminates right around Yellowstone is evidence of Continental Drift. The presence of Basalt in the Pacific Northwest, indicates that the area may have been underwater, as basalt comes from oceanic tectonic plates, not continental plates. Different types of igneous rocks can also tells us about different types of volcanoes, where they are, and with what frequency they may erupt, and that can tell us where the convection currents that drive continental drift are.
A Geologist could tell you even more about what rocks can tell us about what's under the surface of the Earth.