Quantum phase transitions occur when a material changes its state due to quantum mechanical effects rather than temperature. These transitions happen at absolute zero or near it, where thermal fluctuations are minimal. Instead, changes in parameters like pressure or magnetic field can lead to different phases, such as from a conductor to an insulator. In these transitions, the system's ground state alters, often leading to new properties and behaviors. Quantum phase transitions are important in understanding phenomena in condensed matter physics and can be observed in materials like superconductors and magnetic systems.