PubMed provides access to bibliographic information which includes MEDLINE as well as:
The out-of-scope citations (e.g., articles on plate tectonics or astrophysics) from certain MEDLINE journals, primarily general science and chemistry journals, for which the life sciences articles are indexed for MEDLINE.
Citations that precede the date that a journal was selected for MEDLINE indexing.
Some additional life science journals that submit full text to PubMedCentral and receive a qualitative review by the National Library of Medicine."
PubMed may also be searched as part of the NLM Gateway, which also includes resources on consumer health and meeting abstracts.
The National Library of Medicine (NLM) offers an interactive online tutorial about PubMed. Use PubMed Help for quick questions.