Inflation is the leading paradigm for the early Universe. Not only it solves several major puzzles of the Hot Big Bang model, such as the flatness puzzle, the horizon puzzle, and so on, but it offers a natural mechanism to explain the origin of cosmic structures. The quantum fluctuations generated by the inflation field during inflation finally seeded the formation of large-scale structure and the anisotropies of cosmic microwave background observed today. The simplest setup of inflation is the so-called slow-roll inflation model in which the inflation in the early universe is supposed to be driven by the potential energy of inflation field, and ended when the potential became steep and the kinetic energy of inflation field became dominant. In order to solve the puzzles of the Hot Big Bang model, the inflation should last for 60 e-foldings at least, and then the potential of inflation field should be flat enough. On the other hand, since the energy density of the Universe during inflation is dominated by the potential of inflation field, the quantum fluctuations of inflation will finally generate the primordial density perturbations which seed the formation of large-scale structure and the anisotropies of cosmic microwave background radiation. Due to the almost un-evolving Hubble parameter during inflation, the inflation model predicts a nearly scale-invariant power spectrum of density perturbations which has been confirmed by observations at a high confidence level. In addition, the gravitational waves should be excited and finally generated a non-zero power spectrum of primordial gravitational waves. Unfortunately, up to now, the primordial gravitational waves have not been detected. However, the origin of the inflation field and how to embed the inflationary scenario into a fundamental theory are still open questions. In this sense, more studies and more observations are needed before we can finally answer what drove cosmic inflation.