Quantum magnetism is the research field dedicated to investigating the magnetic properties and collective behavior of localized or itinerant electron spins in condensed matter systems where quantum mechanical effects, such as fluctuations, entanglement, and zero-point energy, are dominant. It explores the interactions between spins and their environment, the emergence of exotic ground states (like spin liquids or quantum paramagnets), and quantum phase transitions driven by competing interactions or external parameters, providing fundamental insights into many-body physics and potential platforms for quantum technologies.