The IF of a journal for a given year is calculated as the number of citations to the papers published in the preceding two years, divided by the number of papers published in those years. This measure was originally introduced by Garfield to highlight journals that are hot. The index has been calculated annually ever since and for an ever increasing set of journals. As this indicator is readily available (for a non-negligible fee!), people started to use it as a quality indicator. A journal with a higher IF was (and is) considered better. A paper in a journal with a higher IF was considered better. A researcher with papers in journals with higher IFs was considered better. A department with researchers with papers in journals with higher IFs was considered better.
In a sense everyone was right. Ceteris paribus, a journal with a higher IF has a greater impact (whatever that means) and is therefore better in some sense. Unfortunately the IF is not only about quality, but many other things.
