Several countries require a visa in order to enter. Many times the visa can be acquired at the airport upon arrival, but there are also many countries that require you have a visa before you arrive. Some visas are free, while some will cost you a bit of money. Most visas have a duration that will cover the amount of time you will spend in that country for a normal vacation period but there are usually other options should you decide you want or need to stay longer. Each country will have different requirements for student visas, work visas or tourist visas. Requirements also vary based on your citizenship.
It is important to research the requirements of each country and plan ahead. Many times you will need to surrender your passport to an embassy for them to process your visa. This is better to do while in your home country at the local embassy of the country you want to visit. The last thing you want to do is surrender your passport while in Thailand to the Vietnam embassy! If at all possible, secure your visa ahead of time!
Here are some helpful links to get Visa information: