Giant magnetoresistance (GMR) is a quantum mechanical effect observed in certain materials, where their electrical resistance changes significantly in response to an external magnetic field. This phenomenon occurs in layered structures, typically made of alternating thin films of magnetic and non-magnetic materials. When a magnetic field is applied, the alignment of the magnetic layers affects the flow of electrons, leading to a marked change in resistance. GMR has important applications in technology, particularly in hard disk drives and magnetic sensors. It allows for the development of smaller and more efficient devices by enabling data storage and retrieval at higher densities. This technology has revolutionized the way data is stored and accessed in modern electronics.