The Euro is the currency of 12 countries of Europe. The Euro came into being in January 2002, and is now used by 300 million people in 12 countries. Notes issued by one country and valid in another, and you will not have any trouble taking and using currency between countries.
Countries using the Euro
From 1st January 2008 Cyprus and Malta will also use the Euro.
Britain and the Pound
Britain has not joined the Eurozone. Britain still uses the Pound. Northern Ireland is part of Britain and therefore uses the pound. Ireland is a separate country to Britain, and uses the Euro.