Nihil Obstat is Latin for "Nothing stands in the way [of printing]," and it is issued by the diocesan Censer. Imprimatur is Latin for "Let it be printed," and it is issued by the local bishop.
It is important to realize that the "Nihil Obstat" and "Imprimatur" do NOT mean that a book bearing these labels is inerrant, inspired, or divine. It only means that the local bishop didn't find the book objectionable. I say this because many Protestants try to emphasize the Imprimatur of certain books that they deem heretical in order to show prove their claims against the Catholic Church. Don't be fooled by them.