I always ask question about Third Normal Form in interviews I take.
Q. What is Third Normal Form and what is its advantage?
A. Third Normal Form (3NF) is most preferable normal form in RDBMS. Normalization is the process of designing a data model to efficiently store data in a database. The rules of 3NF are mentioned here
Normalization is very close to concept of object oriented schema’s and it stores one data at only one place by removing all the redundant data. It also helps to draw the schema easier. Normalization comes at the cost of performance.
Reference : Pinal Dave (http://blog.SQLAuthority.com) , DataModel.org.