Starting from January of year 2013 no visas are required for tourists from the CIS member-states, the EU member-states and the Schengen Acquis states. For the visitors from others countries upon arrival it is necessary to apply to the corresponding department of the airport. In general the procedure lasts 15-20 minutes.
Visa can be also obtained prior to the arrival (online) by applying to the Ministry of Foreign Affairs of Armenia: http://www.mfa.am/eVisa.
More detailed information regarding visa provision can be found at the web-page of Ministry of Foreign Affairs.
For the visitors under 18 years old visa is free of charge. By the expiration of the visa dates it should be prolonged at the Passport and Visa Department (Yerevan, 13a, Mashtots Ave).
The visa to Artsakh (Nagorno - Karabakh Republic) is necessary for all foreign country citizens, except for the ones from CIS countries and Argentina.
For more information about a trip to Armenia, you can contact the Embassy or the Armenian representation of your country. http://www.armeniaforeignministry.am/