PubMed - essentially this is Medline Plus...
Always use the Library link to PubMed even though the site is free to use with a simple URL. Why? The Library link embeds the Get It @ SMHS link on citations viewed in the Abstract format and you can then click out to the full text where access is available.
An alternative to reach the full text is to use a bookmarklet.
PubMed Central (PMC) is
A free archive for full-text biomedical and life sciences journal articles. Some (but not all) PMC journals are also MEDLINE journals. PubMed citations are created for content not already in Medline.
PMC International is a collaborative effort between the US PMC, Europe PMC and PMC Canada.
Medline on Ovid
The same citations that may be found in PubMed, but with the helpful Ovid interface.
Ovid segments the Medline database - when searching you may change between different segments. For instance there is a segment for the Epub Ahead of Print and In-Process & Other Non-Indexed Citations segment and others that search all (as in PubMed) or that search a restricted range of years.
Why have two platforms?
Searchers have personal preferences and find one or the other easier to use. Ovid Medline has more search functionality (proximity searching for example). PubMed can be marginally more up to date as data from publishers flows first to PubMed and from there out to other database providers.
Searching in a variety of databases is sometimes required when doing research.
Although there may be considerable overlap between the coverage of different databases, each indexes unique titles and uses unique index thesaurus terms.