This text is aimed at presenting the basic ingredients necessary to understand shell-type supernova remnants, with particular emphasis on their X-ray emission. The other observational domains (radio and optical) are mentioned but not discussed in depth. I do not discuss at all the synchrotron nebulae, such as the Crab Nebula, which are also relics of a supernova explosion, but are driven by the remaining pulsar rather than the mechanical energy of the supernova. After a brief historical introduction, I describe the hydrodynamic concepts used for supernova remnants. Then I describe the Sedov model governing middle-aged supernova remnants, and the coronal conditions under which the X-ray emission is computed. Quantitative formulae are given for the Sedov model, allowing to relate X-ray observations to energy, ambient density and age. A chapter describes the initial (young) and final (old) stages of supernova remnants. Cosmic-ray acceleration at shocks is described, as well as its consequences on supernova remnants. The final chapter deals with statistical issues.