See "MEDLINE, PubMed, and PMC (PubMed Central): How are they different?" for a complete description.
PubMed comprises more than 26 million citations for biomedical literature from MEDLINE, life science journals, and online books. Citations may include links to full-text content from PubMed Central and publisher web sites.
Open Access Science Journals - titles include: PLoS One, PLoS Biology, PLoS Genetics, PLoS Medicine and others.