see this:  http://wokinfo.com/essays/impact-factor/

I also note that this question has been asked here before.  and you can look at these links to see responses earlier given:  

https://www.researchgate.net/post/What_is_difference_between_impact_factor_and_impact_index_of_journals

https://www.researchgate.net/post/Is_SCImago_Impact_factor_SJR_and_Thomson_Reuters_impact_factor_JCR_considered_alternative_to_each_other_or_not