JCR uses citation data to compare peer-reviewed journals. Each year, there are two editions:
Science edition - more than 8,400 journals in 176 subject categories
Social Sciences edition - more than 3,000 journals in 56 categories
It includes 11,549 total journals across 236 disciplines and 81 countries.
There's a slight overlap of the databases, depending on the discipline (e.g., Psychology).
A journal's Impact Factor is intended to measure how often, on average, authors cite moderately recent articles from that journal. JCR includes other measures such as 5-year Impact factors and Eigenfactors, which estimate the percentage of time library users spend with each journal. Journals may also be compared by total citations received; the number of articles; and so on. The Journal Citation Reports are now produced by Clarivate Analytics.
The 2017 version covering 2016 data is now available.
SUBJECT CATEGORIES: Many journals appear in two or three subject categories. There are 8 categories common to both Editions (e.g., History and philosophy of science), but not all journal titles appear in both editions.
Use the Web of Science database (which also includes citation data for resources in the Social Sciences and Arts & Humanities) to find the citation performance of a particular work or author. JCR covers many, but not all, of the journals indexed in Web of Science.