Family based visas are immigrant visas reserved for relatives of US Citizens and Lawful Permanent Residents (Green Card holders). The two steps in applying for lawful permanent residence in the US (a Green Card) are i) not be inadmissible, and ii) establish eligibility within one of the categories by establishing your relationship to a qualifying relative as either a spouse, child, or parent.
Foreign nationals who are inadmissible include those with certain criminal history, diseases, and those who are likely to be a public charge. A public charge is someone who is dependent on the government. An affidavit of support from the sponsor, or a joint sponsor can allow a foreign national who otherwise has no financial means, to not be considered a public charge.
There are two main classes of relative petitions; “immediate relatives” and “preference relatives.” The difference between immediate relative petitions and preference relative petitions is that preference relatives must wait until their “priority date ” (see the visa bulletin for more details) is current before they can apply for an entry visa or before they can apply for adjustment of status in cases where they are already lawfully in the United States.