Impact Factor vs 5-year Impact Factor In ISI Web of Knowledge Journal Citation Report, we can find any journal Impact Factor (IF) and 5-Years Impact Factor (IF5), which is, as far as I get it, the Impact Factor over the last 5 years (well, the name is quite self-descriptive...).

What interests me is the difference between both indexes. I was wondering how to analyse this difference. If IF > IF5, does it means the journal IF is globally increasing? 

And which one is more accurate while evaluating a journal?
The impact factor is calculated over the two previous years (the 2013 number is based on referencing for 2011 and 2012). A high IF indicates that many papers are referenced very quickly (within 2 years of publication). This indicates high turnover rates for the journal. If the longer term value is lower, it could mean what you describe but can also mean that work published has very limited life span. In many fields where publications are slow to produce, in my case they may involve field and longer term experimental work, it is hard to gain many references very quickly and hence the longer term indicator becomes more important and higher. This is also true for small fields where the number of publications is too small for a regular IF to make sense. In field that I am familiar with the five year factor is higher than the ordinary IF bit it will likely vary depending mostly on the nature of the field; if publications have a long or short time span.

There is another factor called the Cited half-life (The median age of the articles that were cited in a year) which in my field often exceeds 10 years indicating the longevity of publications.

For further reading look at Journal Citation Reports