The Journal Impact Factor (JIF) is the calculated by dividng the number of citations received by all of the articles in two previous calendar years of a journal by the total number of articles published in the two previous calendar years of those journals.
Journal Impact Factors can be found in Journal Citation Reports, a subscription based resource that is available through Western Libraries.
Limitations of journal impact factor:
Only calculates citations for publications that are ndexed by Web of Science
Can only be used to evaluate journals within disciplines, not between them
Excludes journals covered by the Arts & Humanities Citation Index becasue their citation patterns are so different
Most journals are in English and from the developed world, although the Emerging Sources Citation Index is an effort to address that