Journal impact factors are calculated and reported annually by Thomson Reuters' Journal Citation Reports (WSU subscription). As of 2014, Journal Citation Reports includes over 8,400 leading scientific journals.
Journals with higher impact factors publish articles that are cited a greater number of times, on average, than journals with lower impact factors. For example, if a journal has an impact factor of 4 in 2013, this means that articles published in 2011 and 2012 were cited 4 times, on average, in 2013.
B.M. Althouse, J.D. West, C.T. Bergstrom, T. Bergstrom. (2008). Differences in impact factor across fields and over time. Journal of the Assoiciation for Information Science and Technology.
G.A. Lozano, V. Lariviere, Y. Gingras. (2012). The weakening relationship between the impact factor and papers' citations in the digital age. Journal of the Assoiciation for Information Science and Technology.
R. Monastersky. (2005). The number that's devouring science. The Chronicle of Higher Education.
P.O. Seglen. (1997). Why the impact factor of journals should not be used for evaluating research. BMJ.
A. Casadevall, F.C. Fang. (2014). Causes for the persistence of impact factor mania. mBio.
Impact factors should not be compared across different disciplines. Different disciplines vary in their citation practices and publication time lag, which can influence the magnitude of journal impact factors. Journals in the life sciences tend to have the highest impact factors, and basic science journals tend to have higher impact factors than clinical medicine journals.
Impact factors should not be used to evaluate individual articles. In any given year, a small proportion of articles published in a journal account for most of that journal's citations; therefore, publication in a high impact journal does not mean that the article is frequently cited. Also, the scientific quality of a study does not always correlate with journal ranking.
Impact factors should not be used to evaluate researchers. It is common for hiring panels, tenure committees, and granting agencies to evaluate researchers based on the impact factors of the journals in which they publish. This practice of rewarding researchers based on publishing in high impact journals can skew scientific progress by encouraging scientists to restrict their research to topics that are more likely to be published in top journals.