Emigration

Emigration is leaving one's country or area to go live in another. Emigration (leaving) from one country is usually followed by Immigration (coming into) into another. The difference between the two is the perspective (or point of view) of which country a person is leaving and which country they are going to. 
The urge to migrate is a part of human nature. People migrated well before we had written history. Emigrants from a place are its diaspora.
People who move around all the time are called nomads.
Reasons.
Emigration happens for different reasons:
International law handles the different groups of migrants differently.