I have been searching for a definition for handler. basics i've understood that "A handler is a piece of code that is called when something happens, and usually takes some action, like generating a response." - (from http://stackoverflow.com/questions/3246200/what-is-an-handler ). But that can be a trigger or a callback. Also in specific an event handler on a low-level , often works by polling a device and waiting for a hardware response.

So, what is the specific role of a handler ( that makes it unique from a trigger or a callback or any other such function ). Do all handlers have similar role ( event handler, file handler , exception handler, error handler )...