A visa petition is the first step in applying to the U.S. immigration authorities for U.S. permanent residence, commonly called a "green card." Let's say you qualify to immigrate through either a family member or a company that wants to hire you. Perhaps, for example, your father is a U.S. citizen or permanent resident. Or perhaps you've been offered a high-level job, and the employer has already recruited for American employees and found none who were qualified, willing, and able.
Can you apply for your green card yet? No, not quite. It's up to your family member or employer to start the process for you. They must do that by filing a form called a "visa petition"—at which time they become your "petitioner," and you become a "beneficiary."
The idea of the visa petition is to prove your petitioner's interest in helping you immigrate, and that the petitioner has the right and ability to do so, based on your relationship and other factors. If, for example, your mother, who is a U.S. permanent resident, filed a visa petition for you, she would need to include a copy of your birth certificate showing that she is really your mother, and of her green card, showing that she is really a permanent resident.
Or, for example, if an employer filed a visa petition for you, it would need to attach proof that a labor certification was granted (itself a lengthy process, called PERM, which involves proving to the U.S. Department of Labor that no U.S. workers are qualified, willing, and able to take the job); proof that it can actually pay the wages it's offering you (such as its tax returns or annual reports), copies of your college degrees if the job requires a certain level of education, and more.
The visa petition also takes care of some other details, like informing USCIS whether you will be continuing with your application through a consulate outside of the U.S. or through a U.S.-based USCIS office.
Once your visa petition has been approved, it serves another important function: It establishes your place on the waiting list, if you're applying in a category where only limited numbers of green cards (immigrant visas) are given out each year. (That applies to almost everyone except the immediate relatives of U.S. citizens and certain highly qualified workers.)
The date your family member or employer sent in the visa petition becomes your "Priority Date," which is like your number in line at a bakery counter. By checking the monthly Visa Bulletin on the State Department's website, you can see which priority dates are currently being served—that is, who is being allowed to move forward with submitting a green card application, as described in Applying for a Green Card (or Adjusting Status).
Immediate relatives, such as the parents, spouse, or minor, unmarried children of a U.S. citizen, not only don't have to worry about Priority Dates, they have an important advantage if they are living in the United States. If they qualify for what's known as "adjustment of status," a procedure in which the entire green card application is done without leaving the United States, they can submit the adjustment of status application to USCIS together with the visa petition. That can save several months of waiting for USCIS to decide on the visa petition. However, this option is not available to everyone—in particular, immigrants who entered the U.S. illegally normally can't use this option.
For more information on preparing and submitting a visa petition, and ultimately the green card application, see How to Get a Green Card (if you're immigrating through family); Fiancé and Marriage Visas: A Couple's Guide to U.S. Immigration (if you're immigrating based on marriage); or U.S. Immigration Made Easy (if you're immigrating through an employer). All three are by Ilona Bray and published by Nolo.
If you'd like to have a lawyer help you deal with the paperwork and other requirements, read the detailed profiles of lawyers in your area in Nolo's Lawyer Directory.