The journal impact factor is a measure of the frequency with which the "average article" in a journal has been cited in a particular year. It is calculated by dividing the number of current citations to articles published in the two previous years by the total number of articles published in the two previous years. The impact factor will help you evaluate a journal’s relative importance, especially when you compare it to others in the same field.
Journal impact factors are searchable with Journal Citation Reports through ISI Web of Knowledge at http://portal.isiknowledge.com/portal.cgi?DestApp=JCR&Func=Frame