PubMed is the free interface for the premier biomedical database, MEDLINE. It was created & is maintained by the National Library of Medicine and contains both primary & secondary literature.
Articles in PubMed are indexed by MeSH (Medical Subject Headings), terms that have specific definitions within the database & help you to create more focused searches.
These are tutorials created by the National Library Medicine to help you get the most out of PubMed.