They are almost synonyms. The FairTax is the most prominent national retail sales tax (NRST) proposal, currently with 40+ sponsors in the U.S. House as HR 25. They have been other NRST proposals, but in general, most people are referring specifically to the FairTax when they say NRST.
An NRST, in general, is a single-stage, single-rate retail sales tax that taxes goods and services only at the point of initial retail sale. Business-to-business transactins are not taxed. The NRST is designed to completely replace income, estate, gift, and FICA taxes. It is completely transparent, there are no hidden taxes.
There is a strong community on FR committed to promoting the NRST/FairTax: the TaxReform ping list. There is also a smaller, but no less vocal, group of detractors who will also frequent realted threads.