This paper has expository nature and it is a survey on the meaning of symmetry for a discrete dynamical system like cellular automata (CA). The main purpose is to give an approximation to the precise concept of internal symmetries of a local rule and to extend them to the cellular automata. Thus like using this scheme to classify the AC, by means of the orbits generated by its groups of symmetry. For didactical reasons some classical results are presented, as well as some relatively new. A few proof are inserted when they are short enough and contribute with clarity.