HttpHandler help us to inject pre-processing logic based on the extension of the file name requested. So when a page is requested, HttpHandler executes on the base of extension file names and on the base of verbs. For instance, if we have different handlers mapped to file extension. We can also map one handler to multiple file extensions. For instance, when any client requests for file with extension ‘GIF’ and ‘JPEG’, handler3 pre-processing logic executes.

HttpModule is an event based methodology to inject pre-processing logic before any resource is requested. When any client sends a request for a resource, the request pipeline emits a lot of events

Modules are called before and after the handler executes. Modules enable developers to intercept, participate in, or modify each individual request.
Handlers are used to process individual endpoint requests. Handlers enable the ASP.NET Framework to process individual HTTP URLs or groups of URL extensions within an application. Unlike modules, only one handler is used to process a request”.

credits : http://www.codeproject.com/KB/aspnet/HttpModuleandHttpHandle.aspx