ournal Impact Factor is from Journal Citation Report (JCR), a product of Thomson ISI (Institute for Scientific Information). JCR provides quantitative tools for evaluating journals. The impact factor is one of these; it is a measure of the frequency with which the "average article" in a journal has been cited in a given period of time.
The impact factor for a journal is calculated based on a three-year period, and can be considered to be the average number of times published papers are cited up to two years after publication. For example, the impact factor 2013 for a journal would be calculated as follows:
A = the number of times articles published in 2011-2012 were cited in indexed journals during 2013
B = the number of articles, reviews, proceedings or notes published in 2011-2012
impact factor 2013 = A/B
(note that the impact factor 2012 will be published in 2013, because it could not be calculated until all of the 2012 publications had been received. Impact factor 2013 will be published in 2014)
Impact factor of Nature, Science and Cell journals can be found on their journal websites.