The citation count for a given publication (article, book chapter, etc.) represents the number of subsequent peer-reviewed publications list it as a reference. The Publication Portal uses the citation counts calculated by the Thomson-Reuters Web of Knowledge service (with permission).
The Impact Factor of a journal is a measure of the "impact" of a journal. The impact factor is calculated by counting the number of citations in a given year (e.g. 2012) to articles published in the previous two years (e.g. 2010-2011), and dividing that by the total number of citable articles published in that period. The Impact Factor was invented by Dr. Eugene Garfield in 1955 and is a trademark of Thomson Reuters.
The impact factors used in the Publications Portal are those calculated and published yearly in the Journal Citation Reports by Thomson Reuters.