LibAnswers

Boolean operators link search terms together.  There are three:  OR, AND and NOT.

Almost any search tool allows Boolean operators, and you will need to use them when constructing a literature search. 

OR

Use OR to find results that contain one term, or the other, or both.  

cost effective OR value for money

This search will find items that include the phrase cost effective, items that include the phrase value for money, or items that include both these phrases, as shown in the diagram below:

AND

Use AND to find results which contain BOTH terms.

cost-effectiveness AND screening AND open angle glaucoma

This search will find items that include all three phrases (cost-effectiveness, screening and open angle glaucoma).  As shown in the diagram below, the AND search will only retrieve items mentioning all these terms (marked by the red triangle):

You can use a combination of AND and OR, along with brackets,  to build a complex keyword search.  For example, this search:

(cost-effective OR value for money OR economic) AND (screening OR screened) AND (open angle glaucoma OR glaucoma simplex)

This search will find items which contain at least one of the synonyms for each concept. For example, it might find me articles with titles like these:

"Economic evaluation of screening for open-angle glaucoma."

"The clinical effectiveness of screening for open angle glaucoma: a systematic review and economic evaluation."

"Targeted screening may be a cost effective way to detect glaucoma simplex."

NOT

You can use NOT to eliminate results which are not relevant. For example, to find articles about open angle glaucoma, but not cataracts, use NOT to omit items that discuss both: 

open angle glaucoma NOT cataracts

As shown in the diagram below, the NOT search will retrieve all results from the green area, and none from the pink area or the overlap area.

WARNING: It is best to avoid using NOT wherever possible, as you may end up losing results which are relevant. If your search results are too large, try adding more relevant terms, using AND.