Many peoples from other countries are willing to settle in america or migrate from their place to america. They cannot simply move from one place to the another. Either you shoul have relatives(parents, children, spouse etc), or to be a student, to do a job, to start a company, to invest money you should have proper reason that is much more strong. They cannot simply say simple reasons.

The peoples will be allowed temporarily visa or permenantly on the basis of the reason for immigration. They have certain visa, humanitarian and parol types. If you need to immigrate to America you need to fullfill all these types and then only the immigration will be possible. For more details see below...