This is known as truncation. If you want to retrieve items about cats, don't truncate the word cat. If you do, you will also retrieve cataclysm, catacomb, catalepsy, catalog, etc., etc. In some databases, such as Lexis-Nexis Academic Universe , the database will automatically search for simple plurals. In the Dataware databases, such as Periodical Abstracts , there is an option on the search screen to tell the database to search for plurals.
Some databases allow for wild cards to be embedded within a word to replace a single character.
You can also limit the number of characters that the wild card symbol represents. The symbols used for truncation and universal characters are different in different databases. To determine the symbols in the database you're using, check the online help screens or ask a librarian.