Family immigration is the process where you are sponsored by your family member for legal stay in the US. Green card gives you legal resident status in the US. With a green card, you can live and work in the US permanently. There are different ways through which you can get a green card. You can get one through employment or through the Diversity Visa lottery program. Family immigration is also possible where you can be sponsored by your US citizen or permanent resident family member for a green card.
Family based green card :
In the family based green card process, as a legal permanent resident, you can sponsor your spouse and unmarried children. But to sponsor your parents or siblings, you need to be a US citizen. You can get a green card by being the immediate relative or as a family member in a preference category. But who is an immediate relative in the family immigration context? You are a immediate relative if you are the child (unmarried and under 21 years old) of a U.S. Citizen or the spouse (husband or wife) of a U.S. Citizen or the parent of a U.S. citizen (if the U.S. citizen is 21 years or older).
Whereas you will fall into the preference category in the family immigration context if you are the unmarried son or daughter (21 years or older) of a U.S. Citizen or a married son or daughter (any age) of a U.S. Citizen or a sibling (brother or sister) of a U.S. Citizen. So being an immediate relative or in the preference category will be an advantage in the family immigration process.
While sponsoring your family member, the US citizen or lawful permanent resident needs to file an Affidavit of Support. The US citizen or lawful permanent resident (green card holder) needs to file Form I-130, Petition for Alien Relative with the concerned authority that is the USCIS. Once the petition is approved, the USCIS sends a notification to the US citizen or permanent resident who filed this petition and to the National Visa Center. There is a limit to the number of family-based immigrants in certain categories every year. So the petition will remain in the National Visa Center until an immigrant visa number is available. But here, the immediate relatives of the US citizen, namely parents, spouse and children have an advantage as they need not wait for this visa number as a number is made available as soon the visa petition is approved. The foreign relative will then be informed by the National Visa Center about the petition received and will also notify as soon as the visa number is available. So when the petition is approved, the foreign relative can apply for an immigrant visa at the US embassy or Consulate in their country which is called Consular processing. Whereas, if the foreign relative is already in the US, he/she can adjust non immigrant status to a permanent resident status as soon as the petition is approved by the USCIS. Here the foreign relative can complete the processing without even having to return to his home country.
You can also get a permanent resident card through special categories of the family immigration namely, being a battered child OR you obtained V non immigrant status OR born to a foreign diplomat in the United States to name a few.