This paper reviews the theory of inflation, focusing on single and multiple fields, with an emphasis on the dynamics of adiabatic and entropy/isocurvature perturbations. It discusses the phenomenology of reheating and preheating after inflation, covering both gravitational and nongravitational aspects of multi-field inflation. The review also explores inflation in theories with extra dimensions and alternative models such as the curvaton scenario and modulated reheating, which generate large-scale density perturbations. Additionally, it examines the observational implications of adiabatic-isocurvature correlations and non-Gaussianity. The introduction provides a historical context for inflation, explaining its role in addressing cosmological problems like the flatness and horizon issues, and the origin of large-scale structure. The paper then delves into the dynamics of inflation, including the standard big-bang cosmology, the problems it addresses, and the concept of inflationary dynamics. It classifies inflationary models into four types: large-field, small-field, hybrid, and double inflation, each with distinct characteristics and potential applications. The review also covers cosmological perturbations, including metric, vector, and tensor modes, and their evolution equations. Finally, it discusses the primordial power spectra and the nature of adiabatic and isocurvature density perturbations.This paper reviews the theory of inflation, focusing on single and multiple fields, with an emphasis on the dynamics of adiabatic and entropy/isocurvature perturbations. It discusses the phenomenology of reheating and preheating after inflation, covering both gravitational and nongravitational aspects of multi-field inflation. The review also explores inflation in theories with extra dimensions and alternative models such as the curvaton scenario and modulated reheating, which generate large-scale density perturbations. Additionally, it examines the observational implications of adiabatic-isocurvature correlations and non-Gaussianity. The introduction provides a historical context for inflation, explaining its role in addressing cosmological problems like the flatness and horizon issues, and the origin of large-scale structure. The paper then delves into the dynamics of inflation, including the standard big-bang cosmology, the problems it addresses, and the concept of inflationary dynamics. It classifies inflationary models into four types: large-field, small-field, hybrid, and double inflation, each with distinct characteristics and potential applications. The review also covers cosmological perturbations, including metric, vector, and tensor modes, and their evolution equations. Finally, it discusses the primordial power spectra and the nature of adiabatic and isocurvature density perturbations.