An automaton is an abstract machine for modeling computer hardware and software. Automata theory is the study of computation models and the computational problems under these models using automata. Automata theory is closely related to formal language theory. There is a hierarchy in computational models (finite languages, regular languages, context-free languages, recursively enumerable languages and so on.) It is one of the oldest topics in computer science yet there are several new results based on automata theory. Nowadays automata are used in many applications in many different forms. We revisit some known results in automata theory and present how a simple language recognizer (i.e., automaton) is being used in several applications.