Most of the above answers cover the key differences. Agencies are self-contained organisations that get their funding directly from a Government, produce their own accounts, have performance targets agreed for them with their Minister, and generally get on with their work without day-to-day interference from the Minister. Agencies are linked to a Government Department, so for example, the Passport Agency is an Agency of the Home Office and the Met Office is an Agency of the MOD. There are hundreds of them across most Government departments. Some Agencies have what is called Trading Fund status. These Agencies are allowed to sell products and services, and raise revenue, and use the revenue generated. So for example the Met Office 'sells' weather info to farmers and commercial organisations, as well as coming on the Beeb every day to tell us its going to be yet another hot day. Agencies that are not linked to a Department (and hence a Minister) are called Non Departmental Public Bodies (or NDPB) but that's probably too much information for you.