Crystal systems are classifications of crystals based on their internal structure and symmetry. There are seven primary crystal systems: cubic, tetragonal, orthorhombic, hexagonal, rhombohedral, monoclinic, and triclinic. Each system is defined by specific axes and angles that describe how the crystal lattice is arranged. The arrangement of atoms in a crystal affects its physical properties, such as hardness, cleavage, and optical characteristics. Understanding crystal systems is essential in fields like mineralogy, materials science, and chemistry, as it helps scientists predict how different materials will behave under various conditions.