Journal impact factor indicates the influence a journal has in a particular field. If the impact factor of a journal is 1.0 - this means that articles published in that journal in the past one or two years have been cited at once. Frequency of citation related to the 'average article' in the journal reflects the popularity and influence of the journal. Bibliometric citation analysis is the most common method of evaluating a journal.
A reliable tool for calculating impact factor is JCR database published by Thomson Reuters Scientific. This database provides a list of journals and assigns an impact factor for each.