A custom service provides the ability to plug into a WebSphere appserver to define a hook point that runs when the server starts and shuts down.
A developer implements a custom service containing a class that implements a particular interface. The administrator configures the custom service in the administrative console, identifying the class created by the developer. When an appserver starts, any custom services defined for the application server are loaded and the server run-time calls their initialize methods.
See AlsoDeveloping custom services