Journal Impact Factor measures the importance of a journal by the number of times its articles have been cited over a particular period. The larger the impact factor number, the more influential the journal.
Journal impact factors vary among disciplines; impact factors only have meaning when comparing journals in the same discipline.
For a quick introduction to the topic of Journal Impact Factor, see a tutorial from Ebling Library for the Health Sciences (University of Wisconsin-Madison).
Journal Citation Reports (JCR) - (GALILEO). JCR is the original source for journal impact factor. It only covers journals indexed by Web of Science, which has relatively weak coverage of the social sciences and humanities. JCR has added several other metrics in addtional to Impact Factor: Immediacy Index, Cited Half-Life, Eigenfactor, and more.
The following sources provide journal circulation numbers:
Association of Business School (ABS) Academic Journal Quality Guide, Version 4 (2010) - "The Guide should be comprehensive in the coverage of research conducted in Business Schools in the UK and internationally covering a wide range of disciplines, fields and sub-fields within the social sciences and taking an inclusive approach to what constitutes business and management research."
Journal Ranking Articles Published 1990-2009, by Discipline - from the Association to Advance Collegiate Schools of Business (AACSB).
The University of Texas-Dallas Top 100 Business School Research Rankings: Tracks publications in 24 leading business journals. The database contains titles and author affiliations of papers published in these journals since 1990.
APA Journal Statistics and Operations Data: Provides manuscript rejection rates, circulation data, publication lag time, and other statistics for journals indexed in the American Psychological Associations databases.
In addition to simple citation counts, more sophisticated measures of researcher impact have been devised. Each has their own advantages and limitations. These are some of the more commonly used measures, but many more have been proposed.
H-index: Measures productivity and impact. An index of h means that your h most highly-cited articles have at least h citations each. It doesn't factor in the length of a researchers career, self citations, and ignores order of authorship. Average impact scores vary widely from discipline to discipline. It has been the most widely used index, but others have been created to address potential weaknesses, including:
The hi-index (AKA individual h-index) takes number of co-authors into account. Your hi-index is equal to your h-index divided by the average number of authors on the articles in your h core.
The hc-index (AKA contemporary h-index) weights newer articles more heavily than older articles, which allows a clearer picture of more recent levels of productivity and impact.
The m-index takes differences in career length into account, by dividing your h-index by the number of years that you have been publishing.
G-index: The g-index gives added weight to highly cited papers, unlike the h-index. An index of g means that your g most highly-cited articles together have at least g-squared citations. Your g-index will always be equal to or greater than your h-index.
i10-index: Developed by Google Scholar, it's the number of publications with at least ten citations.