Given that your questions are very broad and at high level, I would suggest
that you should pick up a book or such to go through that. The Hadoop:
Definitive Guide by Tom White is a great book to start with it.
Meanwhile some links to start with:
On Thu, Aug 8, 2013 at 8:03 PM, Olivier Austina
> I start learning about mapreduce with Hadoop by wordcount example. I am a
> bit confused about the frontier between the map and reduce program. Is
> there a standard format for the map output and the reduce input? Is there
> a full explanation of java classes used somewhere? I also appreciate to
> learning in other languages (phyton, C++ ...). Any suggestion is welcome.