Quantum phase transitions occur when a system changes its state due to quantum mechanical effects rather than thermal fluctuations. These transitions typically happen at absolute zero temperature, where the system's properties shift dramatically as a parameter, like pressure or magnetic field, is varied. In contrast to classical phase transitions, such as melting or boiling, quantum phase transitions involve changes in the ground state of a system. They are often studied in materials like superconductors and magnetic systems, providing insights into fundamental physics and potential applications in quantum computing.