Unitary and federal states
The United Kingdom is a unitary state in that the Westminster Parliament is sovereign and can pass legislation for all four countries of England, Wales, Scotland and Northern Ireland. Devolution has not ended the unitary nature of Britain because the devolved powers are revocable and subject to being overridden by the UK Parliament. The devolved powers are effectively lent, rather than given.[1]
America and Germany are examples of federal states in that under their constitutional arrangements powers are given to the state level. The precise allocation of powers will vary from country to country but typically the federal state will, as a minimum, retain powers relating to foreign policy, defence, immigration and maintenance of a currency.
[1] However, the Scottish and Welsh institutions are expressed to be a permanent part of the UK’s constitutional arrangements and under existing legislation each could only be abolished in Scotland or Wales by a referendum: Scotland Act 1998, s63A and Government of Wales Act 2006, sA1.