Journals metrics are a proxy measure used to evaluate the quality of a journal.
The most commonly used metrics are the:
In Australia, the ERA journal list for 2010 and 2012 is also used as a quality measure.
Journal metrics are used in grant applications, when staff are applying for promotion and when identifying potential journals in which to publish.
Google Scholar has released their own ranking system in April 2012, called 'Google Scholar Metrics'.
For more information see 'Google starts ranking journals'.