An exchange rate is the rate at which one country's currency can be exchanged for other currencies in the foreign exchange. There is no such thing as "the" exchange rate - so if £ depreciated against the $, it could still be appreciating against the Euro or Japanese Yen.
Depreciation
Is a fall in the value of a currency in a floating exchange rate system
Devaluation
Is a fall in the value of a currency in a fixed exchange rate system
Appreciation
Is a rise in the value of a currency in a floating exchange rate system
Revaluation
Is a rise in the value of a currency in a fixed exchange rate system
A free-floating currency
Where the external value of a currency depends wholly on market forces of supply and demand - there is no central bank intervention
A managed-floating currency
When the central bank may chose to intervene in the foreign exchange markets to affect the value of a currency to meet specific macroeconomic objectives
A fixed exchange rate system
E.g. a hard currency peg either as part of a currency board system or membership of the ERM Mark II for those EU countries eventually intending to join the Euro.
