Keiner vs Niemand
what would the difference of keiner and Niemand be? are they used in different context?
They are mostly synonymous. "Niemand" is used exclusively for persons.
Here you cannot replace "keiner" with "niemand": "Keiner der Rasenmäher funktionierte."
"Keiner" can also be used in a relative context, whereas "niemand" is used only absolutely: "Keiner der beiden". Here "niemand" would be strange. But I think that even native speaxers would not notice this kind of nuance most of the time.
I had the same question, and found this discussion useful: http://german.stackexchange.com/questions/15111/keiner-jeder-vs-niemand-alle
Also note that "keiner" is not gender-neutral: http://germannn.tumblr.com/post/44440397858/niemand-vs-keiner