Journal Impact Factors are released annually by Thomson Reuters' Journal Citation Reports (Emory subscription).
A journal's Impact Factor for a particular year = (the total number of times its articles were cited during the two previous years)/(the total number of citeable articles in the journal during those two years).
Within a given research field, journals with higher Impact Factors are thought to be more influential than journals with lower Impact Factors. The magnitude of Impact Factors varies substantially across different research fields.
SCImago Journal Rank (SJR) indicators are released annually at Scimago Labs www.scimagojr.com (free). The underlying data come from Scopus.
The SJR indicator is inspired by Google's PageRank algorithm. A journal's SJR indicator is a measure of the number of citations received by its articles considering the importance of the journals where those citations came from. It is intended to measure journal prestige as opposed to journal popularity.
Calculation of the SJR measure was recently modified; average SJR is now equal to 1, which means that journals with SJRs higher than 1 are more prestigious than average.
The Source Normalized Impact per Paper (SNIP) indicator, created by Henk Moed, corrects for differences in citation patterns among disciplines. A journal's SNIP = (citation count per paper)/(citation potential within its field).
Calculation of the SNIP measure was recently modified; average SNIP is now equal to 1, which means that journals with SNIPs higher than 1 are better than average for their discipline.
Journal Article Influence and Eigenfactor scores are released annually by the University of Washington's www.eigenfactor.org (free). The underlying data come from Journal Citation Reports.
A journal's Article Influence is a measure of the average influence of its articles over the first five years after publication. It is similar in interpretation to Journal Citation Report's Impact Factor.
A journal's Eigenfactor is a measure of the journal's overall importance to the research community. It considers not just direct citations to a journal's articles but rather the entire network of citations that are linked to that journal's articles.