Search field operators called Boolean Operators will help to narrow or broaden out a search. In PubMed, operators should be written in ALL CAPS.

AND 

The AND operator conjoins two search terms. Each of the search terms or associated concepts must be somewhere in the record. You will have fewer search results.

OR

The OR operator links two or more search terms separated with the 'OR'. At least one of the search terms must be somewhere in the record. You will have more search results.

Nested Searching

A search that uses multiple terms, usually with an "OR" separating the terms, in conjunction with an "AND", should contain the OR'ed terms within parenthesis.

Field Descriptions and Tags

One way to narrow a broad search is to add a field tag that indicates to PubMed exactly where you want it to look for your keyword. Two of the most useful field tags are [title] and [title/abstract]. PubMed will only look in those locations for your exact keyword or phrase.