http handler is a information between the web browser and 
web server

An ASP.NET HTTP handler is the process (frequently referred 
to as the "endpoint") that runs in response to a request 
made to an ASP.NET Web application. The most common handler 
is an ASP.NET page handler that processes .aspx files. When 
users request an .aspx file, the request is processed by 
the page through the page handler. You can create your own 
HTTP handlers that render custom output to the browser.
An HTTP module is an assembly that is called on every 
request that is made to your application. HTTP modules are 
called as part of the ASP.NET request pipeline and have 
access to life-cycle events throughout the request. HTTP 
modules let you examine incoming and outgoing requests and 
take action based on the request.

An HTTP handler can be either synchronous or asynchronous. 
A synchronous handler does not return until it finishes 
processing the HTTP request for which it is called. An 
asynchronous handler runs a process independently of 
sending a response to the user. Asynchronous handlers are 
useful when you must start an application process that 
might be lengthy and the user does not have to wait until 
it finishes before receiving a response from the server.