The impact factor of a journal is a quantitative tool for evaluating the relative importance of a journal. It is a measure of the frequency with which its published papers are cited up to two years after publication.
To find out an impact factor use Journal Citation Reports. JCR is available within Web of Knowledge and is the key source of information about the impact of a journal, giving impact factors, cited half-life and immediacy index for each title. JCR covers specialties in the areas of science, technology, and the social sciences and is updated annually in two editions. The Science edition covers over 5,000 journals; the Social Sciences edition covers over 1,500 journals.
A short summary from David Pendlebury of Thomson Reuters (who produce the Journal Citation Reports & Impact Factors).
SCImago Journal & Country Rank uses algorithms similar to Google Page Ranking and is based on data in the Scopus database. Unlike JCR, it does cover some arts and humanities subjects.
The Eigenfactor metrics use a similar methodology to SCImago based on data from the Web of Science database.
More detail about different types of metrics including SCImago and Eigenfactors (from Elsevier, who produce Scopus).
An explanation of how the SNIP & SJR (Scopus Journal Metrics) are calculated.