In the first part of the paper we focus on the different reasons for introducing generalized functions. The second part is devoted to different classes of generalized functions. We follow three directions: Generalized functions as continuous functionals defined on some spaces of smooth functions or on a space of holomorphic functions; cohomo-logical definition; generalized functions as elements of a field or a ring which extend the ring of continuous functions or the ring of smooth functions. We also analyze some properties of the mentioned spaces of generalized functions and the possibilities of solving partial differential equations in them.