Recent advances in quantum technologies have given rise to the field of quantum thermodynamics, which aims to redefine thermodynamic concepts and reformulate thermodynamic laws for quantum systems operating far from equilibrium. Theoretically, this framework modernizes nineteenth-century thermodynamics and extends its scope into the quantum domain. Practically, it enables the design and optimization of energy devices that can surpass classical performance limits. In this paper, we systematically review the fundamental theoretical frameworks and recent research progress in quantum thermodynamics. We first discuss the nonequilibrium dynamics of open quantum systems and the microscopic mechanisms underlying quantum thermalization. Using the exact master equation derived via the Feynman-Vernon influence functional, we analyze the non-Markovian relaxation of a quantum harmonic oscillator strongly coupled to a reservoir. Our analysis reveals that the thermalization behavior is crucially determined by the energy-spectrum characteristics of the composite system. We then elaborate on the two-point measurement scheme for defining thermodynamic quantities such as work, heat, and entropy, and review the corresponding nonequilibrium fluctuation theorems. In particular, we show that a generalized energy-exchange fluctuation theorem can be established for arbitrary system–reservoir coupling strengths by introducing a dynamical effective temperature, which reduces to the conventional Jarzynski–Wójcik equality in the weak-coupling limit. Extensions to entropy production and work fluctuation theorems are also discussed. On the application side, we review several prototypical quantum thermodynamic devices and their underlying physical principles. For quantum thermometry, we show that criticality associated with bound-state formation can overcome the divergence of measurement error at low temperatures, yielding Fisher information that scales as T^-2 over the entire temperature range. For quantum heat engines, we highlight how quantum coherence and squeezing can be harnessed to surpass the classical Carnot efficiency, as exemplified by a photonic Carnot engine and a squeezed quantum Otto engine. We also describe the minimal quantum absorption refrigerator within the framework of a virtual-temperature picture. For quantum batteries, we discuss the collective charging advantage, a bound-state-assisted remote wireless charging protocol that is immune to decoherence, and an NV-center-based scheme that robustly suppresses self-discharging by optimizing the coherent ergotropy fraction. Together, these results illustrate how genuine quantum effects can dramatically enhance the performance of energy devices, and they point toward promising directions for future quantum thermodynamic technologies.