The impact factor is an indicator of journal quality. A measure of the frequency with which the "average article" in a journal has been cited in a particular year, the impact factor is calculated by dividing the number of current citations to articles published in the two previous years by the total number of articles published in the two previous years. Ultimately, the impact factor will help you evaluate a journal's relative importance, especially when you compare it to others in the same field.
Citation Analysis or citation tracking is a way to see the impact of an article in its field. Also, it's a great way to use a "landmark" or influential article to find more recent, related articles that cite the landmark article.