What it might be used for: To evaluate faculty publications for purposes of tenure at some institutions. To inform journal purchase decisions by librarians. To make funding decisions regarding individual researchers, as well as entire institutions, by grantmakers.
Until now the 'Impact Factor' (available from Thomson Reuters' Journal Citation Reports JCR database) was considered the default source for assessing the impact of a journal in terms of the citations it receives.
However, JCR doesn't cover humanities, engineering, or computing very well. It's expensive. And it doesn't address the variations in publishing and citing habits among various disciplines. For example, journals from narrow fields of study generally have lower impact factors.
Lesser known or newer ranking strategies attempt to address some of these issues and they are primarily free. Some promising (and many would argue, more appropriate for evaluating individuals) examples include the eigenFACTOR, SCImago Journal Rank (SJR) and SNIP.
"I am sure that citation statistics of large numbers of individual papers can reflect the impact of contributions at the institutional or national level. They can also provide useful input at the level of an individual. But for a sure assessment of an individual, there is truly no substitute for reading the papers themselves, regardless of the journal in which they appear."
Campbell, Philip [Editor of Nature](2008) Escape from the impact factor.Ethics in Science & Environmental Politics. DOI: 10.3354/esep00078