Many people come to the United States for different reasons such as freedom of religion, good fortune or to find a refuge. They are called refugees because they are forced to leave their homeland to escape persecution or dangers of war, natural disaster or political asylum. They do not feel safe wherever they were living therefore they are seeking protection and shelter in the United States.

For example, in my country Haiti, we had experienced many dictatorships; still today people are trying to leave the country to go somewhere else. Some people may be granted a visa before they settle here they usually hired a lawyer to help them upgrade their status trough the INS. The process is long and may take years. Sometimes it is really sad to see someone not able to go and paid respect to their love one when they have a death in their family.
A large number of Haitians, for emample, are escaping by boat in hope of a better life. They are generally sent back to Haiti.
http://www.cnn.com/2004/WORLD/americas/02/25/haiti.revolt/