SPARC has released Article-Level Metrics—A SPARC Primer.
Here's an excerpt:
Article-Level Metrics (ALMs) are rapidly emerging as important tools to quantify how individual articles are being discussed, shared, and used. ALMs can be employed in conjunction with existing metrics, which have traditionally focused on the long-term impact of a collection of articles (i.e., a journal) based on the number of citations generated. This primer is designed to give campus leaders and other interested parties an overview of what ALMs are, why they matter, how they complement established utilities, and how they can be used in the tenure and promotion process.