A Visa is permission for you to visit and stay in a country for a specific purpose for a specific period of time. It usually must be attached or stamped in your passport.

Information on U.S. Visa formalities
USvisa.com

Generally, a citizen of a foreign country who wishes to enter the United States must first obtain a visa, either a nonimmigrant visa for temporary stay, or an immigrant visa for permanent residence.

The "visitor" visa is a nonimmigrant visa for persons desiring to enter the United States temporarily for business (B-1) or for pleasure or medical treatment (B-2).

Persons planning to travel to the U.S. for a different purpose such as students, temporary workers, crewmen, journalists, etc., must apply for a different visa in the appropriate category. The consular officer can provide additional information.

Travelers from certain eligible countries may also be able to visit the U.S. without a visa on the Visa Waiver Pilot Program.

(Use the link given above to obtain more details)

Australia
http://www.immi.gov.au/allforms/visa_info/index.htm

Anyone who is not an Australian citizen needs a valid visa to enter and stay in Australia.

There are many types of visas, each designed to suit a specific purpose, the length of the proposed stay in Australia and the type of activities people want to do when they get there.

Temporary visas have a time limit on the period of stay in Australia. They are issued with conditions. Some conditions restrict employment or relate to studies. All conditions listed on the visa must be observed or there is a real risk that the visa may be cancelled.

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Canada
http://www.cic.gc.ca/english/coming/imm5256e.html

In order to apply for a Canadian visa you must: have a valid passport; have enough money to support yourself during your visit; intend to stay in Canada for a temporary period only; complete an application form; and pay a non-refundable processing fee at the time you apply.

. (Use the link given above to obtain more details)

Britain
http://visa.fco.gov.uk/

Not all persons wishing to travel to the UK require visas or pre-entry clearance. The country of your nationality; and the country in which you currently reside, or from which you intend to apply determines whether a visa is necessary.

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Switzerland
http://www.swissembassy.org.uk/london_emb/e/home/trach/visares/visreq.html

Each case is reviewed separately at the time of application, and no guarantee can be given that a visa will be issued.

Tourist / Business / Visitor's Visa for stays up to 3 months maximum can be applied. Work (paid or unpaid) or gainful activities are not permitted during the visa period.

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Italy
http://www.esteri.it/eng/foreignpol/italstra/vistieng/homevisas.htm

Entry into Italy from outside is allowed only to those who holds a valid passport or other equivalent travel document, recognised by the Italian Government as valid for crossing borders.

He/She should hold documents which justify the purpose and conditions of the envisaged visit and prove of sufficient financial means for the nature and duration of the visit envisaged and for the return journey to the country of origin.

The person which is to be permitted entry should not be considered a threat to public order, national security or the international relations of one of the Contracting Parties by national provisions or by those of other Schengen States ( Austria, Belgium, Denmark, Finland, France, Germany, Greece, Iceland, Italy, Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden).

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Austria
http://www.austria.org/visa.html

Citizens of the United States in possession of a valid U.S.-passport, are not required to obtain a visa for Austria for a period of up to three months if traveling as a tourist.

In case that a stay of more than three but less than six months is intended, a visa is required.

If you are a resident of the U.S. but do not hold U.S. citizenship, a visa may be required.

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Belgium
http://www.belgium-emb.org/Visitors/Visa/Menu_EN.htm

Nationals of any States are allowed to enter Belgium, provided they have a valid passport or equivalent travel document plus a visa or an authorisation in place of a visa, valid for Belgium.

(Use the link given above to obtain more details)

Denmark
http://www.denmarkemb.org/visa.htm
You may apply for a visa at any authorised mission in a country, where you are staying legally, either permanently or temporarily.

An application for residence work and/or permit has to be submitted to the Danish embassy in your home country or in the country, where you have had permanent and legal residence during the last three months.

The application should be accompanied by a recent photo of good likeness as well as documentation for the purpose of the stay.

(Use the link given above to obtain more details)

Germany
http://www.germany-info.org/relaunch/info/consular_services/visa.html

US citizens in possession of a valid US passport do not need a visa for airport transit, tourist or business trips (for stays up to 90 days).

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United Arab Emirates (Dubai, Abu Dhabi & Sharjah)
http://www.uae.org.ae/tourist/index.htm

Travel to Dubai is now easier than ever with the introduction by UAE Immigration authorities of a visa on arrival facility for 33 nationalities from countries worldwide.

In Europe, these are: Andorra, Austria, Belgium, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Liechtenstein, Luxembourg, Monaco, the Netherlands, Norway, Portugal, San Marino, Spain, Sweden, Switzerland, and the Vatican.

In addition, the new ruling covers nationals from Australia, Brunei, Hong Kong, Japan, Malaysia, New Zealand, Singapore and South Korea in Asia, as well as those from the United States of America and Canada.

If you are a citizen of one of these countries, you no longer need to apply for a visa in advance of travel - a visit visa is now stamped directly in your passport by Immigration officials at the port of entry, and is valid for 60 days.

In addition, if you are traveling on a British passport, you can continue to enjoy the existing visa on arrival facility; if you are resident in the GCC, you can also visit Dubai without the need to apply for a visa in advance of travel. Holders of British overseas passports issued in Hong Kong or China can obtain a visa on arrival at Dubai Airport.

Visa laws and regulations are subject to change without notice. You are requested to contact the UAE embassy in your respective countries for complete details.

(Use the link given above to obtain more details)

Singapore
www.sir.gov.sg

Foreigners holding travel documents issued by the following countries will require either a Business or Social Visit visa prior to entry into Singapore:

Afghanistan, Algeria, Bangladesh, Cambodia**, Commonwealth of Independent States, Egypt, India*, Iran, ,Iraq, Jordan*, Laos*, Lebanon, Libya, Myanmar*, People's Republic of China, Pakistan, Saudi Arabia, Somalia, Sudan, Syria, Tunisia*, Vietnam*, Yemen.

(Use the link given above to obtain more details)

China
http://www.china-embassy.org/eng/6752.html

In accordance with the law of China on the Administration of Entry and Exit of Foreign Nationals, all foreigners must follow certain procedures to apply for a visa and must present required documents.

One needs an actual passport of more than six months validity with a blank visa page. And one complete application form (The People's Republic of China Visa Application Form) with one recent passport size photograph (black and white or color) to apply for the visa.
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Turkey
http://www.turkiye.net/lacg/subdir/passport.htm

Foreign nationals holding passports of the following countries are exempt from visa requirement for stays:
A. Up to three months:
Argentina, Bahamas, Bahrain, Barbados, Belgium, Belize, Chile, Costa Rica, Denmark, Ecuador, El Salvador, Fiji, Finland, France, Germany, Greece, Granada, Hong Kong Special Administrative Region, Ice land, Iran, Israel, Jamaica, Japan, Kenya, Qatar, Kenya, Kuwait, Liechtenstein, Luxemburg, Malaysia, Malta, Mauritius, Monaco, Morocco, Norway, New Zeeland, Oman, St. Lucia, San Marino, Saudi Arabia, Seychelles, Singapore, South Korea, Sweeden, Switzerlan d, Trinidad-Tobago, Tunisia, Turkish Republic of Northern Cyprus, United Emirates, Uruguay, Vatican;
B. Up to two months:
Bosnia-Herzegovina, Republic of Croatia, Indonesia, Macedonia, Romania, Moldova, Republic of Slovenia;
C. Up to one month:
Bolivia, Costa Rica, Kazakhstan, Kyrgyzistan, Maldives, Republic of South Africa.

(Use the link given above to obtain more details)