The document describes three main types of rocks: 1) Igneous rocks form when magma cools and solidifies, either deep underground or at the surface. Examples are granite, basalt, and obsidian. 2) Sedimentary rocks form from compression of sediments and contain layers and fossils. Examples are limestone, sandstone, and shale. 3) Metamorphic rocks form from changes to existing rocks through heat, pressure, and reaction with fluids. Examples are marble, slate, and gneiss.