A note on Journal Impact Factor (JIF)
The Journal Impact Factor was conceived in 1955 by Eugene Garfield (view a collection of Garfield's works here). As one of the first metrics created for evaluating academic journals, it has been instrumental to the field of research impact metrics. As a proxy for the relative importance of a journal within the field, it is typically used to assist authors in selecting a journal for publication.
However, it is important to note that no one metric can tell the full story of a journal's reputation and utility. The JIF has been criticized for being badly skewed by "blockbusters" (i.e. one heavily cited paper can greatly affect the average), for being used inappropriately to evaluate authors themselves rather than journals, and for being too heavily influenced by the citation behaviors of different disciplines (e.g. social sciences tend to cite more slowly and less often than natural sciences).