When searching PubMed you may come across links to articles in PubMed Central (PMC). PubMed Central is a free archive of full-text biomedical and life sciences journal articles. Most, but not all of PMCs content is indexed (searchable) in PubMed.
Although PubMed is essentially a way of accessing the MEDLINE database, it actually contains some additional content, including references to some books and chapters, in-process and ahead of print citations, and some articles which lie slightly outside of the subject scope of MEDLINE, such as those on astrophysics.
Note, however, that WU's access to MEDLINE includes in-process and ahead of print citations so it is unlikely that you would have a need to search both PubMed and MEDLINE. If you are undertaking a systematic review and have decided to run a supplementary search of PubMed in addition to Medline, then consider combining your PubMed search with this search string (pubstatusaheadofprint OR publisher[sb] OR pubmednotmedline[sb]) to reduce duplicate results.
Tip: PubMed is a great interface for carrying out a basic scoping search, or if you wish to identify a limited number of key references. MEDLINE via the links below are recommended if you wish to carry out a comprehensive and systematic search.