Eugene Garfield, the founder of 'Institute for Scientific Information' (ISI) devised the Impact Factor list. ISI, now part of 'Thomson Reuters' evaluates impact factors for various journals that are listed in the 'Journal Citation Reports' (JCR) every year. The JCR report provides a tool for quantitatively evaluating, classifying and comparing journals.
The impact factor for each journal is calculated, which defines the average number of times a paper is cited for two years after its publication. Higher the impact factor for a journal, more influential it is since it measures the mean of paper's citation frequency for a period of time. The list of journals and their IF is used to compare different journals in a particular field.