What are the basic differences between genbank, NCBI and entrez gene? And please tell some books and webpages where I can get more about the databases .(currently I'm using Bioinformatics and functional Genomics by Pavsner)
Genbank is a database of nucleotide sequences that are public.
NCBI stands for "National Center for Biotechnology Information" (in the US).
Entrez is NCBI's retrieval system.
You can find more information in this article
I don't know which kind of databases you are looking for, but there are hundreds of them! In this wikipedia article there are some of them.