Nichols algebras are Hopf algebras in braided tensor categories with very particular properties; for instance, the positive parts of quantized enveloping algebras at a generic parameter, and their finite-dimensional counterparts when the parameter is a root of one, are Nichols algebras. The input datum to define a Nichols algebra is a braided vector space, that is a solution of the braid equation or equivalently of the quantum Yang-Baxter equation. Nichols algebras of diagonal type are by definition those corresponding to solutions of the braid equation given by a perturbation of the usual transposition given by a matrix of non-zero scalars. These Nichols algebras appear in the classification of pointed Hopf algebras with abelian group, within the method proposed by Hans-Jürgen Schneider and myself.