Prestige and Impact http://scientific.thomson.com/knowtrend/essays/journalcitationreports/usingimpactfactor/
A journal's reputation may not tell the complete story about its impact on the scholarly community. In fact, a study by Christenson and Sigelman on social science journals suggests quite the opposite. Their research showed that there is a nonlinear relationship between a journal's reputation and its impact, especially at the extremes of the prestige scale. They conclude that citation data "permit scholars to evaluate the importance of journals based not on opinion but on the frequency of citations" and that "frequency of citation implies scholarly acceptance, or at least acknowledgment of importance through utilization of others' work." The researchers go on to mention that "journals have prestige, but their prestige is only derived from the usefulness of the articles they publish."
The JCR® satisfies the need for quantitative measures. It provides a detailed picture of the scientific literature. It shows the journal-to-journal relationships and permits the discerning user to track important trends or changes over the years, such as a shift from pure to applied research. The changes are not always reflected in the names of the journals. For instance, while the title of the Journal of Experimental Medicine conveys one image, its primary focus today is in fact immunology.