The Journal Impact Factor (JIF) is a measure of the frequency with which the "average article" in a journal has been cited in a particular year. The impact factor will help you evaluate a journal's relative importance, especially when you compare it to others in the same field. The relative importance of a journal may help you decide in which journal to publish your research.
Journal citation reports (JCR) is a database in which you can find the impact factors of journals and other metrics relating to journals. Search by journal name or by subject category. "InCites Journal Citation Reports" can be accessed from within the Web of Science database. Also see the slideshow on Journal Metrics.
Elsevier now provides three alternative, transparent and accurate views of the citation impact a journal makes by using the following metrics:
CiteScore calculates the average number of citations received in a calendar year by all items published in that journal in the preceding three years. CiteScore rank and trend provides a view of the source's rank and percentile for each subject category that it belongs to.
"The SCImago Journal & Country Rank is a portal that includes the journals and country scientific indicators developed from the information contained in the Scopus® database (Elsevier B.V.). This indicator shows the visibility of the journals contained in the Scopus® database from 1996." http://www.scimagojr.com/index.php
The Eigenfactor project aims to "use recent advances in network analysis to develop novel methods for evaluating the influence of scholarly periodicals, for mapping the structure of academic research, and for helping researchers navigate the scholarly literature." http://www.eigenfactor.org/about.php
Eigenfactor metrics are also made available within "Journal Citation Reports" for each journal title.
Scholar metrics provides an overview of recent citations to many publications to help researchers decide where to publish their research. These metrics offer a list of the top 100 publications in several languages. A search within various subject categories and subcategories is also available. To see which articles in a publication were cited the most and who cited them, click on its h5-index number to view the articles as well as the citations underlying the metrics.