PubMed is a free resource developed and maintained by the National Center for Biotechnology Information (NCBI) at the National Library of Medicine® (NLM). PubMed provides free access to MEDLINE, NLM's database of citations and abstracts in the fields of medicine, nursing, dentistry, veterinary medicine, health care systems, and preclinical sciences. MEDLINE is NLM's bibliographic database of citations and abstracts, currently from more than 5,600 biomedical journals published in the United States and worldwide. http://www.nlm.nih.gov/pubs/factsheets/pubmed.html
Check out the Fact Sheet MEDLINE, PubMed, and PMC (PubMed Central): How are they different? to learn more about the difference between these three resources.