Timer Service is a mechanism using which scheduled application can be build. For example, salary slip generation on 1st of every month. EJB 3.0 specification has specified @Timeout annotation which helps in programming the ejb service in a stateless or message driven bean. EJB Container calls the method which is annotated by @Timeout.
EJB Timer Service is a service provided by Ejb container which helps to create timer and to schedule callback when timer expires.