Mapreduce for beginner

Hi,
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.
Thanks.
Regards
Olivier