The three most commonly used operators are AND, OR and NOT.   They can broaden or narrow your set of results and exclude unwanted search terms and concepts.

For example:

AND will narrow your search returning results that contain all of your search terms 

myocardial infarction AND aspirin

OR will broaden your search returning results that contain any but not all of your search terms. It is useful for finding synonyms or where different words are of equal value in your search 

heart attack OR myocardial infarction

NOT will narrow your search by eliminating words from your search results. It should be used with care as it can easily exclude relevant results.

myocardial infarction NOT silent

To search for two or more words in exact order, place double quotation marks " " around the words. The database will only return articles containing that specific phrase rather than articles containing each word found individually anywhere in the text.

Example:

The phrase "heart attack" will retrieve articles with all words as you typed them in with no other words in between.   

Truncation is also known as stemming, is a technique that broadens your search to include alternative word endings and spellings.

To use truncation, enter the root of a word and put the truncation symbol * at the end.
The database will return results that include any ending of that root word.
For example:
child* = child, child's, children, children's, childhood

It is important not to shorten the root too much as it may retrieve too many irrelevant results. For example chil*  will bring up childless, chiller, chilly, and Chile.