What is PubMed?
PubMed is a database service provided by the National Library of Medicine (NLM). The primary database that PubMed provides access to is called MEDLINE, also created by the NLM. In addition to MEDLINE, PubMed provides access to a variety of medical trade journals, cross listing into other NLM databases, and access to full-text content uploaded to PubMedCommons as part of the open access requirement for NIH grants.
In other words, "PubMed is one way to access MEDLINE."
Why use one or the other?
The benefit to PubMed is that it contains information beyond what is inside the MEDLINE index. It also includes publisher provided pre-print citations that may take months to reach the MEDLINE index. The downside to PubMed is that our FINDIT button that provides a link into the full-text doesn't work in PubMed. Unless the article is available free from PubMedCommons you will have to manually check our library holdings to see if we own the journal. Information on how to do so is provided in the Guide to PubMed. EBSCO's MEDLINE does have the FINDIT button, but is missing the extra content. Which database you should you depends on how comprehensive of a search you wish to do and how familiar you are with navigating the library.