Truncation is like a wildcard. Added to the root of a word, it will find that root plus anything that comes after it. The symbol used to truncate a word depends upon which resource you are using. Below are two standard truncation symbols used by many databases and some search engines. If these don't work in the resource you are using, check the Help button and look for the term truncation or wild cards.

Some search engines, like Google and Yahoo, will do what is called stemming (which is very similar to truncation because it looks for plurals and grammatical variants) automatically for most words that you search.

psychol? will return records on psychology, psychological, psychologist.
? is used as the truncation symbol, for example, in ABI/Inform, a very popular article database in the field of business.
environ* will return records on environment, environments, environmental.
* is used as the truncation symbol in Academic Search Premier and InfoTrac OneFile, two general article databases.