Journal Impact Factors (IFs) are used to help evaluate and compare scholarly journals in the Sciences and Social Sciences. IFs are an index of the rate at which papers published in a journal are subsequently cited in a given year.
ISI Journal Citations Reports is the main source of journal impact factors.
IFs are considered a quantitative and objective measure of the quality of journals and, increasingly, by implication, the scholars who publish in them (the higher the IF the higher the quality). However, there are a number of reasons not to take IFs at face value. Depending on the lag time in publishing, size of the discipline and how self-contained it may be, the extent to which the discipline depends on recently published material, and a number of other factors, IFs have received varying degrees of support and rejection in the academic world.
If you are interested in exploring IFs further, then review the critical literature in your field regarding journal evaluations. To research IFs in your discipline, do keyword searches on the phrase “journal impact” or “impact factor” in relevant indexes listed in the Indexes by Subject page.