Fundamentally, journal metrics measure the number of citations for articles published in a specific journal over a specific period of time. These are always calculated values from databases such as Scopus and Web of Science; many other databases provide this information and so do many journals websites. There are many journal metrics and some common journal metrics are explained below.
Journal Impact Factor:
"The journal Impact Factor is the average number of times articles from the journal published in the past two years have been cited in the Journal Citation Reports (JCR) database year" (JCR, 2014).
5-Year Journal Impact Factor:
The average number of times articles from the journal published in the most recent 5 years have been cited.
Journal Immediacy Index:
The average number of times articles from the journal published in the current year have been cited.
SJR - SCImago Journal Rank:
The measure of a journal's scientific influence based on the number of incoming citations and the importance of the journals those citations come from with citations from high ranking journals carrying more weight than those of lower ranked journals.
SNIP - Source Normalized Impact per Paper:
"measures contextual citation impact by weighting citations based on the total number of citations in a subject field." - Scopus
IPP - Impact per Publication:
"measures the ratio of ciations in a year to scholarly papers published in the three previous years divided by the number of scholarly papers published in those same years." -Scopus
Eigenfactor uses the number of incoming citations to measure a journal’s importance to the scientific community with citations from higher ranking journals weighted to make a larger contribution than those of poorly ranked journals - i.e. Are the articles in your journal of interest being cited by high impact or low impact journals?
There are many usages for the journal impact factor. Here is a list of some of the common usages:
Scopus is an interdisciplinary database that is used to calculate SJR, SNIP, and IPP.
Journal Citation Reports (InCites) provides the journal impact factor, 5-year journal impact factor, and journal immediacy index.