From the home page:
PubMed is a service of the U.S. National Library of Medicine that includes over 17 million citations from MEDLINE and other life science journals for biomedical articles back to the 1950s. PubMed includes links to full text articles and other related resources.No registration is necessary to access PubMed, and searches can be based on topics, authors or journals. The list of results will show whether an abstact is available for each article. Complete articles, if not available online, can be read in university and public libraries. Sign up for an account and you'll receive email updates and alerts for new content.