What is US' newly-created Department of Government Efficiency?

Donald Trump has appointed Elon Musk and Vivek Ramaswamy to lead a new Department of Government Efficiency (DOGE). It isn't an official government agency and will advise the White House on reforms. It'll partner with Office of Management and Budget to drive structural changes, bringing entrepreneurial approach to government efficiency. Its name references Musk's favourite cryptocurrency, Dogecoin.