Quantum sensing represents a transformative approach to precision measurement, enabling detection at the atomic scale by leveraging fundamental quantum phenomena. By utilizing the unique properties of quantum systems—such as superposition, coherence, and entanglement—these sensors achieve exceptional sensitivity and accuracy, surpassing the limitations of classical sensing technologies. This paper explores the core aspects of quantum sensing, beginning with an overview of the principles and platforms that enable it. Various systems—each with their respective advantages and limitations—are examined in detail. The review also delves into quantum sensing protocols, highlighting how measurement sequences are governed by the system’s Hamiltonian. Particular emphasis is placed on key applications, especially in the biomedical and energy sectors, showcasing existing technologies and their impact. The paper concludes by addressing current challenges, including coherence maintenance and system integration, while also outlining promising future directions that could shape the development of next-generation quantum sensing technologies.