In essence it really comes down to a few things
- Contains all articles from all of the 5400 journals in indexed by PubMed whether biomedical or not (approx. 1 million citations)
- Contains all non-indexed articles and OLDMEDLINE
- It is the free search interface for the various databases associated with the National Center for Biotechnology Institute
- Site specific tools, i.e. MyNCBI, etc.
- Is the major database component in PubMed (approx. 21 million citations)
- Only contains fully indexed biomedical journal citations
- Is usually leased out to other database providers, i.e. OVID, EBSCO and isn't updated nearly as often as when it is used through PubMed
There is a great article about this from the National Library of Medicine located here http://www.nlm.nih.gov/pubs/factsheets/dif_med_pub.html.