Visas are required for the citizens of all countries except Thailand. However, you can obtain a visa on arrival at Vientiane and Luang Prabang international airports, as well as at the Friendship Bridge from Nong Khai in Thailand. It is generally cheaper to obtain the visa on arrival than to obtain it at a Lao embassy abroad. The exception is Bangkok, where a visa will cost about a third less at the embassy that it does at the border. The drawback is that the Lao embassy in Bangkok is in a rather obscure part of town, and the time it takes to get a visa can be two days or more.
To get your visa on arrival, you'll need a passport size photo of yourself and at least US$ 35 cash. The cost of the visa varies according to the country of your passport. Visas are generally good for 30 days.