There are three basic types of rocks found on Earth.
Igneous rocks are created when liquid or molten stone, called magma or lava depending on if it's above or below ground, cools and hardens. Igneous rocks are often formed by volcanic actions. An example of Igneous rocks in the Pacific Northwest are those found on the slope of Mt. St. Helens in Washington.
Sedimentary rock is formed by many layers of sand and silt (or sediment), hardening into rock. Often sedimentary rocks are formed from ancient sea floors, lake or river beds or shorelines, where sediment piles up over a long time. Fossils are often found in sedimentary rock. Sedimentary rock make up many of the layers or stripes of rock in the Grand Canyon.
Metamorphic rocks are other rocks that are changed by heat and pressure into a new kind of rock. For example, shale transforms into the metamorphic rock slate. These rocks are often found in mountain ranges like the Rocky Mountains or the Appalachian Mountains, which used to be very large, but are now just the remaining metamorphic rocks that formed their core.