A citation index allows the user to establish which later documents cite which earlier documents. The impact factor is a measure reflecting the average number of citations to articles published in science journals. It is frequently used as a proxy for the relative importance of a journal within its field.
Both the Citation index and Impact factors are used to measure how good an article is. This is a misunderstood and a misleading way of using such measures. Unfortunately institutions and governments use citation index to fund researchers.
Recently, the PLoS and BioMedCentral journals give authors some information on citations, but also tell you how many times the article has been downloaded. This is called article-level metrics, and measures individual papers.
However, it is important to recognize that also such metrics are prone to errors. An article that is mentioned in a newspaper, is likely to be downloaded more often that articles that do not receive such media attention.
Although these new metrics provide us with more information, it is not yet clear how best to use these measures. Currently, both PLoS and BioMedCentral simply present the data.
But, there is only one way of deciding whether a paper is important and to you, and that is to read it, critically analyse it, and come to your own conclusions.