The map function outputs〈target, source〉 pairs for each link to a targetURL found in a page named source. The reducefunction concatenates the list of all source URLs as-sociated with a given target URL and emits the pair:〈target, list(source)
as i said eariler i think i am sliglty more confident now
The map func-tion processes logs of web page requests and outputs〈URL, 1〉. The reduce function adds together all valuesfor the same URL and emits a 〈URL, total count〉pair.
Till now what i understood is that - Put in raw data to map func it give structured key value of what you need - Give huge amount of structured key value generated from map to reduce func and it will summarize (for eg) and return thr output to you like total count of all the occurnces etc