"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:

• Use MeSH to Build a Better PubMed Query
• PubMed: Using the Advanced Search Builder

Some comments::

• When you search PubMed, look at the documentation about it. For example, look at these sections in the table of contents.

Searching for a phrase

Truncating search terms

Combining search terms with Boolean operators (AND, OR, NOT)

• When you search a topic in the MeSH database (MeSH=Medical Subject Headings), you can limit the search to aspects of the topic. Say you want to find literature diagnostic imagining of the brain. You can search on "Brain" in the MeSH, then check off "diagnostic imaging" in the list of subtopics.  
• If you notice that MeSH is not picking up the most recent results, if there is a delay in assigning MeSH headers, try other search approaches.