A Green Card is the United States document for permanent residency and allows immigrants to live, work, study and travel throughout America for as long as they wish.
There is a certain amount of time that a permanent resident must remain within the United States before they can apply for citizenship, but there is no such requirement for people who wish to adjust status to permanent resident.
Technically, if you are applying for permanent residency as an immediate relative of a United States citizen, you could potentially become a permanent resident as soon as you enter the country under consular processing.
Immigrants who claim another class of family based immigration may have to wait a significant amount of time between when their petition is accepted and when a visa number comes up that will let them come to the United States. In this situation, these immigrants are allowed to visit the United States for this period of time, but they cannot live continuously in the country while their visa number is backlogged.
Unlawful presence in the United States for whatever reason will end up further complicating any immigration process. In some instances immigrants face bars on their entry for up to 10 years for unlawful presence in America.
Many people choose to use the H-1b non-immigrant visa to immigrate to the United States. While usually non-immigrant visas do not allow their holders to immigrate to America, H-1b allows dual intent and therefore the freedom to immigrate on an employment class Green Card. There is no really required period of time that one must be in the US on this visa before they can apply for a Green Card, but there is a limitation to how many times you can renew this visa before you must return to your home country.
There are many people who already live in the United States without proper immigration paperwork, these people are often referred to as “illegal”, but really they are only without documentation and therefore “undocumented.”
As the law stands right now, undocumented residents of the US are required to leave the country for a specific amount of time before they will be allowed to file for immigration to America. However, there has recently been talk in the United States Congress about allowing undocumented immigrants the option to get work permits and eventually a path to permanent residency and citizenship. However, this legislation is not yet solid.
There is a program available right now for people who were taken to the United States as children that allow them to file for work permits called DACA (Deferred Action for Childhood Arrivals.