"PubMed® comprises more than 30 million citations for biomedical literature from MEDLINE, life science journals, and online books. Citations may include links to full-text content from PubMed Central and publisher web sites." From PubMed.
Here is a National Library of Medicine's (NLM) tutorial about searching the PubMed interface. PubMed is a freely accessible search database for medically related literature. Although PubMed is free on the web, make sure you access it off Lehigh's library homepage, since you will then see Lehigh Links (mentioned in the Web of Science tutorial).
In the NLM tutorial, the presenter shows you how a PubMed search is mapped into a controlled search vocabulary (thesaurus). The terms in this vocabulary are used in PubMed to index articles, therefore providing a very focused way of searching. You can search in PubMed itself or go directly to the MeSH database.
Some other tutorials are these: