It was first mooted by former education secretary Estelle Morris in 2001 as part of the Transforming secondary education agenda, and given flesh by her successor Charles Clarke in the 2002 Education Act. Schools set up long-term arrangements to collaborate closely on school improvement.
Ministers envisage a whole spectrum of arrangements. At one extreme, in "hard" federations, schools will draw up contracts to work together.
Usually, they will share leadership, perhaps under an executive head, and governance - either under a single governing body or a joint committee of governors from each.
"Soft" federations, meanwhile, involve less formal arrangements, that could involve shared staffing, curriculum development or staff training. The DfES so far has 11 federations running with others in development.