Added to the stem of a word, a truncation symbol will find that stem plus anything that comes after it. The symbol used to truncate a word depends upon the database or Web search engine you are using. 

will return records on market, marketing, markets.? is used as the truncation symbol, for example, in ABI/Inform - Dateline and the library catalog.
will return records on pig, pigment, pigmentation, pigments, pig
will return records on advertise, advertised, advertising, advertisement, advertisements.
* is used as the truncation symbol in InfoTrac OneFile and Wilson Select Plus.