In any introductury course on quantum mechanics we are told that the information about any properties of some physical system is contained in its quantum state |psi>. However, there are occasions when we do not have full access to the information about the state and then we need to introduce the concept of density matrices. In this lecture I will present the basic ideas about such density matrices. In particular, I give examples where they naturally appears, like for thermal states or what is called entangled states. A related topic that will be discussed is that of information.
