The impact factor is one way of measuring the amount of influence a journal has within the scientific community. It is primarily used by the indexing service provided by the Institute for Scientific Information, known as ISI Web of Knowledge (now part of Thompson Reuters).
 Calculating the IF
The factor is calculated as the number of citations in a given year to articles published within the previous two-year period. The 2008 impact factor of a particular journal would be calculated as follows:
A = the number of times articles published in 2006 and 2007 were cited by indexed journals during 2008 B = the total number of "citable items" published in 2006 and 2007. ("Citable items" are usually articles, reviews, proceedings, or notes; not editorials or Letters-to-the-Editor.) 2008 impact factor = A/B
Science and Nature are considered two of the most prestigious journals of all time. These have impact factors of 29.78 and 30.98 respectively. It should be noted, however, that 90% of all the citations to Nature are to do with only a quarter of its published papers. Also, despite these being the "best" journals in the eyes of most researchers, they are not the top ranked according to impact factor, several journals dedicated to publishing reviews come above these two journals - and indeed, the highest ranked journals tend to specialise in publishing reviews as these types of articles tend to attract more citations than normal research papers.
The magnitude of the impact factor is also discipline specific, with certain areas of science attracting more attention. So while the highest ranking journal that specialises in agriculture has an IF of about 2.5, the highest in molecular biology is 33.4.
While it is a good proxy for the quality of a scientific journal and shows a journal's ability to distribute important information and findings widely, the IF is not a be-all and end-all figure or a completely flawless method of analysis. The IF is open to manipulation, as was seen when the journal Folia Phoniatrica et Logopaedica published an editorial that cited all of its papers from 2006–2007; thus bumping its IF from 0.66 to 1.44 in a single step. This was primarily a protest against the use of the IF as a pure measure of a journal's quality. Similarly, Mohamed El Naschie's habit of citing his own papers in the rag Chaos, Solitons, and Fractals (of which he was the chief editor) caused that journal to have a higher impact factor than any other mathematics publication.
That many researchers and editors attach too much importance to the impact factor is something that is not denied by Thompson Reuters, and they advise caution in using it because of the number of phenomena that can lead to significant changes in a journal's IF regardless of quality.