PubMed is the free interface for the premier biomedical database, MEDLINE. It was created & is maintained by the National Library of Medicine. PubMed contains both primary & secondary literature. Because it's a free to access, you can use it even when you leave the University of Michigan.
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.