What is a 'Journal Impact Factor'?
"Impact Factor: The measure of the frequency with which the average article in a journal has been cited in a particular year. The impact factor will help you evaluate a journal’s relative importance, especially when you compare it to others in the same field. It 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." JCR information and help.
Impact Factor is one indication of the journal's relative importance and prestige. It may be useful when selecting a journal in which to publish. i.e. a journal with a high impact factor.
Web of Science and Scopus also provide information related to a journal's publishing impact. e.g. Scopus via the Journal Analyzer provides a SJR which is a measure of the scientific prestige of scholarly sources. However, you need to consult Journal Citation Reports (JCR) for the Impact Factor.
The 'H-Index' - is a measure of an author's publishing impact.
"The h-index is based on a list of publications ranked in descending order by the Times Cited. The value of h is equal to the number of papers (N) in the list that have N or more citations." Web of Science help.
Web of Science and Scopus provide h-index data, however the calculation may differ between the databases, as based on different journals and years covered by each database.
Google Scholar also provides h-index and metrics data. Individual researchers need to set up a profile in Google Scholar:
Google Scholar Citations: http://scholar.google.com.au/intl/en/scholar/citations.html