This review provides a pedagogical introduction to the eigenstate thermalization hypothesis (ETH), its foundations, and its implications for statistical mechanics and thermodynamics. The first part introduces ETH as an extension of quantum chaos and random matrix theory (RMT). It discusses classical and quantum chaos, RMT, and key predictions such as energy level statistics, eigenstate components, and matrix elements of observables. ETH is shown to describe thermalization in isolated chaotic systems without requiring an external bath. Numerical evidence from many-body lattice systems and results from quantum quenches are presented. The second part explores the implications of quantum chaos and ETH for thermodynamics, deriving fundamental thermodynamic relations, fluctuation theorems, and the fluctuation-dissipation relation. It shows how quantum chaos allows proving these relations for individual Hamiltonian eigenstates, extending them to arbitrary ensembles. The review also discusses relaxation dynamics and description after relaxation in integrable systems, where ETH is violated. It introduces the generalized Gibbs ensemble (GGE) and its connection to prethermalization in weakly interacting systems. Keywords include quantum statistical mechanics, eigenstate thermalization, quantum chaos, random matrix theory, quantum quench, quantum thermodynamics, and generalized Gibbs ensemble. The review covers topics such as classical chaos, RMT, quantum chaos in physical systems, eigenstate thermalization, thermodynamics, fluctuation theorems, and integrable models. It discusses the relation between quantum chaos and thermodynamics, the role of ETH in deriving thermodynamic relations, and the behavior of systems after quenches. The review also addresses the challenges of achieving thermalization in isolated systems and the importance of ETH in understanding thermalization in quantum systems.This review provides a pedagogical introduction to the eigenstate thermalization hypothesis (ETH), its foundations, and its implications for statistical mechanics and thermodynamics. The first part introduces ETH as an extension of quantum chaos and random matrix theory (RMT). It discusses classical and quantum chaos, RMT, and key predictions such as energy level statistics, eigenstate components, and matrix elements of observables. ETH is shown to describe thermalization in isolated chaotic systems without requiring an external bath. Numerical evidence from many-body lattice systems and results from quantum quenches are presented. The second part explores the implications of quantum chaos and ETH for thermodynamics, deriving fundamental thermodynamic relations, fluctuation theorems, and the fluctuation-dissipation relation. It shows how quantum chaos allows proving these relations for individual Hamiltonian eigenstates, extending them to arbitrary ensembles. The review also discusses relaxation dynamics and description after relaxation in integrable systems, where ETH is violated. It introduces the generalized Gibbs ensemble (GGE) and its connection to prethermalization in weakly interacting systems. Keywords include quantum statistical mechanics, eigenstate thermalization, quantum chaos, random matrix theory, quantum quench, quantum thermodynamics, and generalized Gibbs ensemble. The review covers topics such as classical chaos, RMT, quantum chaos in physical systems, eigenstate thermalization, thermodynamics, fluctuation theorems, and integrable models. It discusses the relation between quantum chaos and thermodynamics, the role of ETH in deriving thermodynamic relations, and the behavior of systems after quenches. The review also addresses the challenges of achieving thermalization in isolated systems and the importance of ETH in understanding thermalization in quantum systems.