For a citizen of a foreign nation to apply for and receive an immigrant visa, U.S. immigration law generally requires that the person be petitioned (sometimes called sponsored, though this actually has a narrower meaning). A family relation or an employer can petition for someone.
Petitioning for someone typically involves filing an initial visa petition on their behalf, typically using USCIS Form I-130 (in family cases) or I-140 (in employment cases). It also requires some support through the rest of the process.
Family petitioners become "sponsors" as well, because they must promise the U.S. government to support the immigrant financially (and show capacity to do so). If the green card interview will be held in the United States, a family petitioner may possibly need to attend the interview along with the immigrant.
Employer petitioners must promise to provide a job when the immigrant arrives, at a certain salary. (However, the employer does not have to promise to keep that person working there for any particular length of time.)
There are two types of family-based immigrant visas: immediate relatives and preference relatives.
Immediate relative immigrant visas include a U.S. citizen's:
Family preference immigrant visa categories include:
The preference level of a requested visa will determine how long the immigrant has to wait until a visa becomes available. Immediate relatives wait the least time -- they have an unlimited number of visas available to them, so it's just a matter of getting through the application process. Preference relatives' wait times tend to get longer as the family relationship gets more distant. Brothers and sisters of U.S. citizens face the longest waits, often up to 24 years.
Which country the relative is from also affects wait times. There is a per-country limit on visas, so when there's high demand, the wait gets longer. The longest waits tend to affect people from Mexico, the Philippines, India, and China.
Employers may also petition for immigrant employees to receive green cards in order to work with them. (Don't confuse this with the many temporary visas that also require employer petitions, such as an H-1B.) Some immigrating workers don't need an employer to petition for them, however.
For an employer to bring in a foreign citizen to work, it must get labor certification from the U.S. Department of Labor. This involves proving that no U.S. workers are ready, willing, and available to take the job being offered. Then the employer will file a petition called the Immigrant Petition for Alien Worker (Form I-140) with USCIS. There are five categories for these types of visas.
Again, the different preference levels can impact the difficulty or ease of receiving a visa, and how long one has to wait.
Applying for an immigrant visa can be confusing for both petitioner and immigrant alike. And the stakes are high -- one mistake and a case can be delayed for months, or fail altogether. To ensure you select the proper visa for the proper type of immigrant and petitioner, and for help assembling all required documentation and completing the various forms, it is always advisable to seek advice from an immigration attorney.