From the website summary: "Formative assessment is intended to yield real-time information about if and how students are learning—and what might be impeding their progress. Insights gained from this ongoing classroom assessment process can help teachers shape instruction and guide student efforts as well. The effective use of formative assessment can add balance to assessment systems that, in recent years, have leaned heavily toward assessment for accountability purposes."
This paper provides a great introduction to formative assessment and practical guidance for implementing it. It also details the research underpinning formative assessment and addresses approaches for measurement validity.