MEDLINE is a subset of PubMed. When using the PubMed database, it is searching for citations on MEDLINE as well as journals and manuscripts deposited in PubMed Central, and the National Center for Biotechnology Information (NCBI) Bookshelf. PubMed is more user-friendly than MEDLINE but not all items in PubMed have been assigned MeSH terms. Because of this, PubMed is a good place to start a general search but for a subject search, try MEDLINE for a more specific match.
For a more detailed explanation on how these databases differ, check out https://www.nlm.nih.gov/bsd/difference.html
Tutorials and Helpful Guides to Using PubMed and MEDLINE