A visa is a travel authorization that gives you temporary entry into another nation. Most of the time, you must apply for a visa in advance of traveling, either online or at an embassy or consulate. A visa may occasionally be obtained upon arrival. Typically, visas are attached to your passport and specify the length of your stay.
For security reasons, most nations require visas for visitors in order to monitor who enters and prevent illegal immigration. Additionally, visas are used defensively to prevent security risks from entering a country.