PubMed is a free interface for searching MEDLINE, the most popular bibliographic database in the health and medical sciences. It contains references to millions of journal articles from biomedical journals. You can access PubMed directly. UCL staff and students can access PubMed via the UCL Library Services databases list in order to see FindIt@UCL links to UCL subscribed journals.
Although PubMed is essentially a way of accessing the MEDLINE database, it actually contains some additional content, including references to some books and chapters, and some articles which lie slightly outside of the subject scope of MEDLINE.
NLM have produced a web page outlining the main differences.
Note, however, that UCL's access to MEDLINE via OvidSP includes in-process and "ahead of print" citations so it is unlikely that you would have a need to search both PubMed and MEDLINE via OvidSP. PubMed is a great interface for carrying out a scoping search, or if you wish to identify a limited number of key references. MEDLINE via OvidSP is recommended if you wish to carry out a comprehensive and systematic search. See LibrarySkills@UCL guide to OvidSP.