Every country has different visa regulations - how long you can stay the price for the visa and the extension requirements are different for each country and change from time to time. Many countries in Southeast Asia allow for a Visa on Arrival - which means you can show up at the airport and get your Visa there. However, some countries require you to apply for a Visa at an Embassy first. Also keep in mind that a Visa on Arrival will generally be for a shorter period of time (1 month or less). If you need to stay for a longer time then you can either apply for a longer Visa at an Embassy before you arrive, or else check if you can apply for a Visa extension once in the country.
Some Visa tips things to keep in mind:
If you plan to enter the country more than once (maybe to take a quick hop over to a nearby country and back) then you will need a multiple-entry visa.
Some countries (such as Vietnam) require that you list you entry and exit points for each of your visas, which can make changing your travel plans difficult.
The type of Visa (and your allowed length of stay) may be different if you arrive by air or by land. Thailand, for example, grants a 30 day visa if you arrive by air, but only a 15 day visa if you enter by an overland crossing.
Some Visas or activated on entry and some are activated on issue. If you get a 1-month Visa that is activated on issue and don't arrive in the country for three weeks, then you will only be able to stay for 1 week.
Cambodia: 30 day Visa on arrival for most nationalities
Malaysia : 60 day Visa for most western nationals, 30 days for US
Singapore: Singapore tourist visa is not required for citizens of United States for a stay up to 30 days.
Thailand : A tourist visa can be used to stay in Thailand for up to 30 days or up to 60 days and can be extended by 30 days at the nearest Immigration office.
Vietnam : Visas are generally issued for single or multiple entries with a validity of one month or three months depending on the fee and at the discretion of the consulate.
The very term Southeast Asia was invented by American Naval strategists around 1940.
For at least two thousand years to the present day, Southeast Asia has been a conduit for trade between India and China. Large-scale Chinese immigration only began with the advent of the colonial era. In Singapore, the Chinese form a majority of the population, but there are substantial Chinese minorities, assimilated to varying degrees, across all countries in the region.
Southeast Asia has a tropical climate. The weather hovers around the 30°C mark throughout the year, humidity is high and it rains often.
The equatorial parts of Southeast Asia, including Malaysia, Brunei, Indonesia, Singapore and the Philippines, have only two seasons, wet and dry.
Southeast Asia is religiously diverse. Malaysia, Indonesia and Brunei are predominantly Sunni Muslim country, while East Timor and the Philippines are predominantly Roman Catholic. In the northern Southeast Asia, Buddhism dominates, mostly of the Theravada Buddhism. However in Vietnam, Mahayana Buddhism dominates.