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Medline is the largest component of PubMed and consists primarily of citations from journals selected for MEDLINE; articles indexed with MeSH (Medical Subject Headings) and curated with funding, genetic, chemical and other metadata.
PMC is a full text archive that includes articles from journals reviewed and selected by NLM for archiving (current and historical), as well as individual articles collected for archiving in compliance with funder policies.
The final component of PubMed is citations for books and some individual chapters available on Bookshelf.
Bookshelf is a full text archive of books, reports, databases, and other documents related to biomedical, health, and life sciences.