Wildcards are special characters which stand for ANY letter in the alphabet. The exact symbol used to represent a single variable letter varies from database to database, but common signs are ?, *, #, and $. To find which symbol your database uses--click on Help and search for truncation.
|psychology, psychological, psychiatry, psychotic, etc.
A truncation symbol is a specific type of wildcard used to represent any number of letters (including zero) at the end of a word. An asterisk is the most common truncation symbol, but check the database help if it doesn't work.
Truncators allow you to put the stem of a word in and pull in all results with different "endings", as shown above.
Not all search engines allow wildcards or truncation (the most famous example is Google). And some automatically searching for common variants of each word, particularly plurals.