What's the difference between PubMed and...
Medline is the largest subset of PubMed. PubMed is larger and more up-to-date as it includes citations to articles that have been not yet been assigned MESH headings, have been supplied directly by the publisher, or are outside of the scope of Medline journals.
PubMed Central is PubMed’s repository for freely available full text articles. When you search PubMed, you are searching the contents of PubMed Central as well as the proprietary literature. When you see the notation "Free Article" in a PubMed citation, that means the article is from PubMed Central.
For more information, see the NLM Fact Sheet MEDLINE, PubMed, and PMC (PubMed Central): How are they different?
PubMed is much larger than CINAHL. PubMed goes back further in time and makes a point of indexing the worldwide literature of the health sciences, while CINAHL (Cumulated Index to Nursing and Allied Health Literature) emphasizes nursing and the allied health disciplines. In addition to journal articles, CINAHL includes books, book chapters, dissertations, and computer programs.
There is an overlap in the journals indexed by these two databases. When searching CINAHL, you can limit your search to material NOT indexed by Medline by completing your search, then selecting the Edit option and then "exclude Medline records."