Montenegro and Kosovo are countries that are not in the EU but use the Euro (Kosovo also uses the Serbian Dinar)
The euro is the current Italian currency. Italy is a member of the European Union. Use of the euro isn't a requirement of EU membership. For example, Sweden is an EU member that doesn't use the euro. Italy is an example of an EU member that does.
Denmark did not leave the EU. It is still a member. It does not use the Euro, but that does not mean it is not in the EU. Some other EU countries do not use the Euro. Only 19 of the 28 member countries use it.
The euro is the unit of currency used by many EU member nations. It replaced each country's own currency, and as such the euro doesn't need to be exchanged when travelling between countries. The German mark, French franc, Austrian schilling, Irish pound, and Italian lire were all replaced.
Spain and Portugal both joined the European Union at the same time and were the 11th and 12th member states.
The EU is just the European Union. Any countries of Europe can apply to receive EU membership if they qualify. The EU (or Euro) zone is the collection of member countries which share the currency the Euro. The Euro is now compulsory for all EU members, except the UK and Denmark which have opted out but may join in the future.
The European Union's main currency is the Euro. All member states have adopted the Euro except for Great Britain and Denmark.
Europe, Malta is an EU member state, in the Schengen zone and in the Euro.
Barcelona is one of Spain's major cities and as Spain is a member of the EU, Barcelona's currency is the Euro.
The Euro 2008 complement is based on UEFA membership (the governing body of European football), of which Turkey is a member, rather than EU membership. Membership with UEFA is independent of affiliation with the EU. 25% of the 16 teams who advanced to the Euro 2008 finals were not from EU member states: they are Turkey, Switzerland, Russia, and Croatia. Turkey and Croatia have made applications for EU membership, Switzerland and Russia have not.
Tenerife is part of the Autonomous Community of the Canary Islands which is part of Spain, a member of the EU. Therefore Tenerife is a Spanish island where the locals speak Spanish and the local currency is the Euro like elsewhere in Spain and the European Union.
Croatia will become a member of the EU on July 1, 2013.
EU law is used in many countries, often in place of a country's own law. Countries are bound by directives made by the EU. Countries using the Euro have to keep with certain economic constraints. Countries are subject to the European Court. So there are lots of ways the EU has influence within a country that is a member that affects a country's decision making.