There are three main types of rocks: sedimentary, igneous, and metamorphic. The document describes how to identify samples of different rocks and determines their classification. Sedimentary rocks form from compressed sediments and often contain layers and fossils. Igneous rocks usually form from volcanic activity and are made of cooled lava with interlocking crystals that lack layers or fossils. Metamorphic rocks were previously sedimentary or igneous rocks that were altered by heat and pressure and may show signs of recrystallization.