There are usually two basic types of search queries: Keyword and Subject (MeSH).
PubMed, however, searches using a third option--a combination keyword/subject search. PubMed does this automatically behind the scenes.
Keyword Searches
Allows you to enter a search term that you believe best describes your topic. While keyword searches may retrieve what you've entered, you also need to search using synonyms and search term variations to make sure that you have retrieved everything. For example, if you are looking for information on "heart attack" using a keyword search, you also need to search using the terms "heart attacks," "myocardial infarction," "myocardial infarctions" and so on.
With a keyword search, you may also retrieve items that just mention your word--but are not necessary about that term.
Subject Searches (MeSH)
Before a record is added to a database or catalog, its subject matter is determined. Specific terms that apply to the record are chosen from a standard list of vocabulary terms and attached to the record as subject terms. So no matter what terms the author uses, one can consistently retrieve information on a concept using subject terms. For example, the term "heart attack" is always listed as "myocardial infarction" within MeSH (Medical Subject Headings)--which is used in MEDLINE and PubMed.
Note that subject terms change from database to database. Some (MeSH is one of these) are even hierarchically ordered--thus allowing you to narrow or broaden your topic.
Combination Subject / Keyword
PubMed uses something called term mapping--basically a combination subject/keyword search. Term mapping is automatic--it's working behind the scenes; you won't see it unless you look for it. Term Mapping takes your keywords and attempts to "map" or match them to the best subject or MeSH term. PubMed then pulls those mapped MeSH terms into your keyword search, and runs a combination keyword/subject search. When it works, it's pretty slick--and honestly, the only database I'm aware of that does this. When it doesn't work, well....your search will fail.
To make sure that term mapping's done a good job, give a click on Advanced, then click on your your search terms. It'll open up a box and show you the MeSH terms that it's mapped and pulled into your search.
Adapted from Valpariaso University.