Journal metrics provide a quantitative method for evaluating the quality of journals and is based on bibliometric citation analysis, where the popularity of a journal is reflected by the number of citations related to the "average article" in the journal.
Various citation indicators have been developed to reflect perceived quality. Each of these metrics has its own particular features, but in general, they all aim to provide rankings and insight into journal performance based on citation analysis.
This is based on the premise that a citation to a paper is a form of endorsement. The number of citations which a particular paper attracts is therefore regarded as an indication of its "importance".
Keep in mind that different journal metrics use different methodologies and data sources, thus offering different perspectives on the scholarly publishing landscape, and bibliometricians use different metrics depending on what features they wish to study.
The three main sources which exist for citation analysis are:
Thomson Reuters’ Web of Science
Journal Citation Reports (JCR) rank journals based on citation data. JCR Science edition provides impact factors for over 7,300 journals in science and technology. JCR Social Science edition provides impact factors for over 2,200 journals in the social sciences.
Scopus Journal Analyzer also ranks journals based on citation data. Scopus indexes more than 17,000 journals across most disciplines.