A crystal structure is defined as the repeating arrangement of atoms in a crystal lattice. There are two main types of solids: crystalline solids, where atoms are arranged in a systematic pattern, and non-crystalline solids, where atoms have a non-systematic pattern. A unit cell is the basic repeating structure of a crystal lattice and shows how atoms are arranged at different sites in three dimensions. There are seven main crystal systems that are classified based on the lattice constants and angles of the unit cell. Common crystal structures include simple cubic, body-centered cubic, face-centered cubic, and hexagonal close-packed.