Journal Citation Reports is published annually in two editions:
JCR Science Edition contains data about more than 8,000 journals in science and technology.
JCR Social Sciences Edition contains data about more than 2,600 journals in the social sciences.
The year that you select is the JCR year. All of the data that you see for journals and subject categories come from journal data published in that year.
For example, if you select JCR Science Edition 2012, and you search for a particular journal, you will see the 2012 data for that journal, including:
Number of articles published in the journal in 2012
Number of citations to that journal from articles published in 2012
Impact Factor calculated from 2012 data, and so
Concerns with impact factor:
New record: 66 journals banned for boosting impact factor with self-citations, 19 Jun 13 Nature News Blog
Impact, more about research and less about factor, 21 June 13 BMC Blog
Alternative to the journal impact, 3 June 13 KTH
San Francisco Declaration on Research Assessment (DORA), 16 Dec 12
Demise of the impact factor, 8 June 12 LSE Blog