Authorative and non-authorative answers..

[Amir]

Can anyone please clearify something ? when i nslookup , i get a list of
ns's which i can get
authorative answers from, and a list of Ns's i can get non-authorative
answers from.
what does that mean ? does that mean the authorative is the master(primary)
server ? (if so , why do i see 2 names there sometimes ? can there be two
masters ????)
thanks , amir