Entrez is the text-based search and retrieval system used at NCBI for the major databases, including PubMed, Nucleotide and Protein Sequences, Protein Structures, Complete Genomes, Taxonomy, and others.
PubMed: The biomedical literature (PubMed) 
Nucleotide sequence database (Genbank) 
Protein sequence database 
Structure: three-dimensional macromolecular structures 
Genome: complete genome assemblies 
PopSet: population study data sets
OMIM: Online Mendelian Inheritance in Man
Taxonomy: organisms in GenBank 
Books: online books 
ProbeSet: gene expression and microarray datasets
3D Domains: domains from Entrez Structure
UniSTS: markers and mapping data
SNP: single nucleotide polymorphisms
CDD: conserved domains
http://www.ncbi.nlm.nih.gov/Entrez/
NCBI
Database Interlinking