After getting an approved immigrant petition, an immigrant must then obtain a "green card," or legal permanent resident card. This can be done in one of two ways: (1) consular processing or (2) adjustment of status. In either case, the alien must have an approved immigrant petition and must not be inadmissible. They must also have an immediately available visa, either because they are an "immediate relative" of a U.S. citizen or because their number is available under the visa bulletin. They must also undergo a medical examination and obtain certain vaccinations. In both cases, they can bring their spouse and unmarried children under 21 years of age with them.
If an immigrant is outside the U.S., they will apply for their visa at a U.S. embassy or consulate through the U.S. Department of State. I'm not entirely acquainted with the requirements, but I understand they have to prove they don't have a criminal record which makes them inadmissible and they also have to prove their identity. If the consular officer approves the application, this still doesn't make them a permanent resident. They must go to a U.S. port of entry (by land, sea, or air) and apply for admission through an officer of Customs and Border Protection, part of the Department of Homeland Security. This officer will also verify their identity and can turn them away if the officer believes they are inadmissible. If the CBP officer admits them, they are then a permanent resident.
Adjustment of Status
If the alien is inside the U.S. (either on a nonimmigrant visa or as an illegal alien who is admissible due to asylum status or a previous amnesty program) they can apply to adjust their status to that of a permanent resident by filing Form I-485 with U.S. Citizenship and Immigration Services, another branch of DHS. The requirements are essentially the same as for consular processing, but they must also establish they entered the country legally and have remained legal the entire time they were here, with some room for forgiveness, or they are eligible for a waiver or meet an exception to the rules.
So, just so we're clear, I'll go over the basic steps again. First, someone in the U.S. (a citizen, permanent resident, or business) petitions for the alien. If the petition is approved, and when the visa is available, the alien makes an application to either a consulate or USCIS. If the application is to USCIS, approval means they are a permanent resident. If the application is to a consulate, then they must come to the U.S. and apply for admission by a CBP officer. Approval makes them a permanent resident.
Next time, I'll discuss some other ways to become a permanent resident which are less common, but also important: refugee and asylum, and the diversity visa program.