Short answers, pulled from the story.
SCImago Journal Rank measures the prestige of scholarly journals by accounting for both the number of citations a journal receives and the prestige of the journals those citations come from. A citation from a highly-ranked journal contributes more to a journal's SJR score than a citation from a lower-ranked one.
SJR was developed by the Scimago Lab, which originated from a research group at the University of Granada. The indicator is free to access.
SJR is based on the Scopus database. This distinguishes it from the Eigenfactor score, which uses the Web of Science database.
Nature ranks first on the SJR indicator. Science ranks second and the New England Journal of Medicine ranks third.
SJR is inspired by, and uses an algorithm similar to, PageRank. Both are based on eigenvector centrality, where connections to high-scoring nodes contribute more to a node's own score. In SJR, journals iteratively transfer prestige to each other through citations until a steady-state solution is reached.
Studies of methodological quality and reliability have found that the reliability of published research in several fields may actually decrease with increasing journal rank, contrary to widespread expectations.