MEDLINE: a National Library of Medicine (NLM) journal citation database. Articles indexed in MEDLINE have been through a review and recommendation process. MEDLINE uses NLM controlled vocabulary, Medical Subject Headings (MeSH) to index citations.
PubMed: PubMed contains all references from MEDLINE, plus additional citations, such as those published before MeSH, certain primary general science and general chemistry journals that are out-of-scope for MEDLINE, as well as author manuscripts, and some books from the NCBU Bookshelf. If you limit your results to MeSH subject terms, you will only see MEDLINE journals.
PubMed Central (PMC): a free archive for full-text biomedical life science journals. It is a repository for journal literature deposited by participating publishers. There is some overlap with journals in PMC and MEDLINE.
Adapted from the U.S. National Library of Medicine.