Warning: This Answer is BIG
Well by default your application goes to all countries (read later in the post and I will post all the countries names). Buy if you don't localized your application (like in Russia they speak Russian, and if your application is only in English the app is useless for only Russian speaking users).
But what if I want my Application only running in America or etc:
Q: How to localize my app with Xcode 4?
A: Check out this links: How to localize my app with Xcode 4?, http://www.raywenderlich.com/2876/how-to-localize-an-iphone-app-tutorial, and http://www.youtube.com/watch?v=oMHRYDEbgpg :)
The Official list of Countries as of May 18, 2012: Link to official list: http://support.apple.com/kb/TS3599 (scroll down to view more countries :)
Algeria
Angola
Anguilla
Antigua & Barbuda
Argentina
Armenia
Australia
Austria
Azerbaijan
Bahamas
Bahrain
Barbados
Belarus
Belgium
Belize
Bermuda
Bolivia
Botswana
Brazil
British Virgin Islands
Brunei
Bulgaria
Canada
Cayman Islands
Chile
China
Colombia
Costa Rica
Croatia
Cyprus
Czech Republic
Denmark
Dominica
Dominican Republic
Ecuador
Egypt
El Salvador
Estonia
Finland
France
Germany
Ghana
Greece
Grenada
Guatemala
Guyana
Honduras
Hong Kong
Hungary
Iceland
India
Indonesia
Ireland
Israel
Italy
Jamaica
Japan
Jordan
Kazakhstan
Kenya
Kuwait
Latvia
Lebanon
Lithuania
Luxembourg
Macau
Macedonia
Madagascar
Malaysia
Mali
Republic of Malta
Mauritius
Mexico
Montserrat
Moldova
Netherlands
New Zealand
Nicaragua
Niger
Nigeria
Norway
Oman
Pakistan
Panama
Paraguay
Peru
Philippines
Poland
Portugal
Qatar
Romania
Russia
St. Kitts & Nevis
St. Lucia
St. Vincent & The Grenadines
Saudi Arabia
Senegal
Singapore
Slovakia
Slovenia
South Africa
South Korea
Spain
Sri Lanka
Suriname
Sweden
Switzerland
Taiwan
Tanzania
Thailand
Trinidad & Tobago
Tunisia
Turkey
Turks & Caicos
Uganda
United Arab Emirates
United Kingdom
United States
Uruguay
Uzbekistan
Venezuela
Vietnam
Yemen