Academies are state-funded schools which, instead of being supported by the local authority, are funded and controlled directly by central government and run by academy trusts - not for profit private trusts governed by company law.
Academies are set up through a contract (a funding agreement) between the Education Secretary and the trust. The trust has legal responsibility for the staff, assets and land of the school(s) it runs – so staff become employees of the trust.
The government is pressing schools to become academies as part of a multi-academy trust (MAT). This is where a trust oversees several schools under a single funding agreement. MATs range in size, but can be very large and are sometimes referred to as academy chains. In practice, schools can no longer convert to become single academy trusts.