Eigenfactor.org is a project of Bergstrom Lab at the University of Washington. The goal of Eigenfactor is to rate a journal's importance to the scholarly community. Towards that goal, Eigenfactor gathers data from around the web, including Thomson Scientific's Journal Citation Reports, to compute two numbers: Eigenfactor score (EF) and article influence (AI).
Eigenfactor score (EF) is a measure of the "journal's total importance to the scientific community."
Aritlce influence (AI) is "a measure of the average influence of each of its articles over the first five years after publication."
For more information on the methodology and data sources behind the rankings, see Eigenfactor.org's FAQ.