difference b/w isapi mappings and http handlers?

Please can anybody help me understand the difference between ISAPI extensions and Http handlers. To my knowledge both are used to allow file extensions (types) to go through the IIS for processing. But waht is the difference. I have gone thru google but no good source. Can yu explain?
Who will process the request either ISAPI or HttpHandler, and what is worker process ? , what it will do other than isapi or httphandler?
Is there any similarity b/w worker process , isapi and HttpHandlers
I am very much confused with all these topics when i try to read Application Life Cycle.