With the exception of visitors from India, Bangladesh and Maldives, all other visitors to Bhutan need a visa and the Indian, Bangladeshis and Maldivian nationals can obtain a visa at the port of entry on producing a valid passport with a minimum of 6 month validity (Indian nationals may also use their Voters Identity Card (VIC)).
All other tourists must obtain a visa clearance prior to travel to Bhutan and the visas are processed through an online system by local tour operator or through a foreign tour operator such as Outfitter Nepal.
You need to send the photo-page of your passport to your tour operator who will then apply for your visa to enter Bhutan. The visa will be processed by the Tourism Council of Bhutan (TCB) once the full payment of your holiday (including a USD $50 visa fee) has been wire transferred and received in the TCB bank account.
At your point of entry you will be required to show your visa clearance letter, the visa will then be stamped into your passport.