The Institute of Science Information (ISI) was founded in 1960 to help index the ever growing collection of scientific journals. The founder of ISI, Eugene Garfield, developed a simple impact factor to give a rough estimate of quality and help highlight the more important journals. Roughly the impact factor of a journal in year x is the average number of citations each article from years x-1 and x-2 receives in year x.
Thomson Scientific bought out ISI in 1992 and turned the data collection into a business. Impact factors are not only being used to measure the quality of journals but of authors (i.e. researchers) and institutions as well. In many parts of the world a person's research quality is being measured strongly or sometimes solely by their ability to publish in high impact factor journals.
This is bad news for computer science since conference proceedings in our field have historically more prestige than journals. We mitigate the ISI factors pretty well in the US but in many other countries this puts computer science at a disadvantage. The need for impact factor publications is one of the reasons conferences are experimenting with a hybrid model.
In a new trend I get many announcements of journals highlighting their ISI impact factor, mostly very general and previously unknown to me. Our old friends WSEAS say "The ISI Journals (with Impact Factor from Thomson Reuters) that publish the accepted papers from our Conferences are now 32" in the subject line of their email.
It's the vanity journal with a twist: publish with us and you'll raise your official numerical prestige. So we get a set of journals whose purpose is to raise the value of researchers who should have their value lowered by publishing in these journals.
Raise your research standing the old fashioned way: Do great research that gets published in the best venues. The numbers will take care of themselves.