Journal Impact Factor
The Journal Impact Factor (JIF or IF) for a journal is calculated as the mean number of citations received in a given year by all articles published in that journal over the preceding two years.
It is based on citation data from the Thomson Reuters ISI Web of Science database, and is obtainable from the associated Thomson Reuters database Journal Citation Reports, provided that your institution has a subscription to this resource.
SJR (SCImago Journal Rank)
The SJR for a journal is calculated in much the same way as the Journal Impact Factor, but with one additional feature. Calculation of the SJR gives greater weight to citations received from articles in more prestigious journals than to citations received from articles in less prestigious journals.
This metric is based on citation data from the Scopus database. It is freely obtainable on the Web from the relevant part of the SCImago web site .
SNIP (Source Normalized Impact per Paper)
The SNIP for a journal is calculated in much the same way as the Journal Impact Factor, but takes particular account of how the number of citations received by a journal relates to the citation potential in its subject field.
In other words, that different academic fields have different average levels of citation, and factors this into the calculation of the metric.
Detailed information about the calculation of this metric is available in an overview page and in open access papers reporting the original development of the metric and some further refinements of it.