What is Normal mapping, How it works, where this Normal Mapping is used …. and lot more question about Normal mapping is answerd in this link this is very good article on Normal mapping from Wikipedia.
Their is also very good explaintion of Normal mapping on this site
you will also find some info from this site [...]

“This is a short tutorial on how to produce a normal map within Maya. It is also intended to be a very simple way to familiarize a person where normal maps are produced within Maya”.
for futher information on Normal mapping look for other article in this blog