Journal impact measurements are usually based on citations and thus reflect the relative importance of a particular journal in a field. They take into account the number of articles published per year and the number of citations to articles published in that journal. Like any metrics, journal impact measures do not consider all factors that make a journal important to follow, and researchers in a discipline will have the best sense of the top journals in their field. What's impact factor? Watch video.
Citation Counts and Journal Rankings Competing Services:
An author's impact on their field or discipline has traditionally been meausred using the number of times their academic publications are cited by other researchers. There are numerous algorithms that account for such things as the recency of the publication, or poorly or highly cited papers. While citation metrics may reflect the impact of research in a field, there are many potential biases with these measurements and they should be used with care. For a critique of author impact factors, see this article in the Chronicle of Higher Education.