Although I have never seen a definition, the term handler is usually applied to something which is not of itself an autonomous function, but which fits into a function to provide a service.

As an example, look at the Apache Web Server. The server software provides hooks where you can interpose code for things such as authentication, authorization, request handling and others. Each of these is known as a phase becuase one pphase starts and completes before another phase can commence. Within the phase there is a provision ofr external code to be used to perform a function in a different way to that which Apache provides itself. The handler code will receive a set of data or structures, process them, and returns a response. For example you might use several types of authorization methods on your web site. Some areas might be protected by Basic .htaccess controls, others might require validation against a database. So you have two handlers. One for the .htaccess method, if it fails Apache will keep checking with any handlers it has been told about until it finds on e that fits, say the database handler, when it returns a success the process is completed and the server moves on to the next phase.

The Apache handler based system is well documented on the Apache web site and in the various books on the server.

Hope that helps
jdtoronto