- If you are a temporary worker/student under a work of student-visa you are considered a 'non-immigrant' since it's temporary and you have the intention to move back to your homeland.
- If you want to settle in the United States and the petition is granted you are an immigrant and considered a 'permanent resident' and you receive the well-known 'Green Card'.
- If you want to become a United States citizen you have to live in the United States as a 'permanent resident' for at least 3 and usually 5 years.
The biggest difference between a Permanent Residency and Citizenship is that with the latter you can't be deported, can vote in ALL elections, run for office (except the Presidency) and it can't be taken away from you.
I will apply for Citizenship in 3 years, but since both the US and the Netherlands allow it, I will retain my Dutch Citizenship and will have dual US and Dutch Citizenship.
Hope that clears it up a little... ;-)