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. The impact factor will help you evaluate a journal's relative importance, especially when you compare it to others in the same field.
Caluculation of impact factor in 2010 *
Cites in 2010 to articles published in:
2009 = 336
2008 = 805
Sum = 1141
Number of articles published in:
2009 = 106
2008 = 103
Sum = 209
Cites to recent articles (1141)
Number of recent articles (209) = 5.459
* Source: ISI Web of Knowledge Journal Citation Reports. Updated 25 November 2011.