Journal impact factor is a measure of the influence that a particular journal has in its field. Researchers often want to publish in journals that have a higher impact factor, so that their individual articles have a higher chance of being seen and referenced. The journal’s impact factor does not necessarily indicate anything about the particular authors publishing in it, except that it may be more prestigious to publish in a higher impact factor journal.
The Information for New Users section provides helpful tutorials and information on “using JCR wisely.” In particular, “you should not depend solely on citation data in your journal evaluations. Citation data are not meant to replace informed peer review. Careful attention should be paid to the many conditions that can influence citation rates such as language, journal history and format, publication schedule, and subject specialty." Impact factor is highly dependent upon discipline, and should only be used to compare journals within a specific area of study.
Web of Science does not index every journal published, and therefore not every journal will have an impact factor.
Eigenfactor is another way that the impact of journals is measured (www.eigenfactor.org). It is based on an algorithm that evaluates the networks between journals. The Eigenfactor attempts to calculate a percentage estimate of the total time that all journal users spend with that journal compared to all other journals. The numbers are normalized so that the total of all Eigenfactors is 100. This number is also available in Journal Citation Reports.