Quantum sensing uses quantum systems, properties, or phenomena to measure physical quantities. It includes historical sensors like superconducting quantum interference devices and atomic clocks, and modern platforms such as spin qubits, trapped ions, and flux qubits. The field offers high sensitivity and precision, with applications in various scientific areas. This review introduces quantum sensing principles, methods, and concepts from an experimentalist's perspective. Key topics include quantum sensors, examples like neutral atoms, trapped ions, Rydberg atoms, atomic clocks, and solid-state spins. The review discusses quantum sensing protocols, sensitivity, noise, and detection methods. It covers time-dependent signals, noise spectroscopy, dynamic range, adaptive sensing, ensemble sensing, and quantum-enhanced sensing using entangled states. It also explores quantum sensors assisted by auxiliary qubits and future developments. Quantum sensing protocols involve initializing the sensor, interacting with the signal, and reading out the final state. The quantum sensor Hamiltonian includes internal, signal, and control components. Sensitivity depends on the transduction parameter and decoherence time. Quantum sensing can detect weak signals with high precision, using techniques like Ramsey and Rabi measurements. The review also discusses the Heisenberg limit, entangled states, and quantum error correction. It highlights the potential of quantum sensors in various applications, including magnetic field sensing, atomic clocks, and high-precision measurements. The review concludes with an outlook on future developments in quantum sensing.Quantum sensing uses quantum systems, properties, or phenomena to measure physical quantities. It includes historical sensors like superconducting quantum interference devices and atomic clocks, and modern platforms such as spin qubits, trapped ions, and flux qubits. The field offers high sensitivity and precision, with applications in various scientific areas. This review introduces quantum sensing principles, methods, and concepts from an experimentalist's perspective. Key topics include quantum sensors, examples like neutral atoms, trapped ions, Rydberg atoms, atomic clocks, and solid-state spins. The review discusses quantum sensing protocols, sensitivity, noise, and detection methods. It covers time-dependent signals, noise spectroscopy, dynamic range, adaptive sensing, ensemble sensing, and quantum-enhanced sensing using entangled states. It also explores quantum sensors assisted by auxiliary qubits and future developments. Quantum sensing protocols involve initializing the sensor, interacting with the signal, and reading out the final state. The quantum sensor Hamiltonian includes internal, signal, and control components. Sensitivity depends on the transduction parameter and decoherence time. Quantum sensing can detect weak signals with high precision, using techniques like Ramsey and Rabi measurements. The review also discusses the Heisenberg limit, entangled states, and quantum error correction. It highlights the potential of quantum sensors in various applications, including magnetic field sensing, atomic clocks, and high-precision measurements. The review concludes with an outlook on future developments in quantum sensing.