Ordinarily, in order to apply for and obtain an immigrant visa and permanent resident status ("green card"), you need a petitioner (although there are several exceptions to this general rule). The petitioner can be either your U.S. employer (employment-based) or your U.S. citizen or permanent resident family member (family-based). But not every employer or family member can be a petitioner; only those employers and family members that fit one of the specific categories set out by the immigration law can file a petition for you.
Family-based Categories
1. IR (Immediate Relatives of U.S. citizens): Immediate relatives of U.S. citizens are not subject to annual quota and therefore there is no waiting time between the U.S. citizen's petition and the relative's application for an immigrant visa or adjustment of status. In other words, immediate relatives' priority dates are always current. Immediate relatives are:
----- U.S. citizen's spouse.
----- U.S. citizen's unmarried children under 21.
----- Parents of adult U.S. citizen (21 or older).
2. F1: U.S. citizen's unmarried sons/daughters who are 21 or older. Subject to quota.
3. F2A: Permanent resident's spouse and unmarried children under 21. Subject to quota.
4. F2B: Permanent resident's unmarried sons/daughters who are 21 or older. Subject to quota.
5. F3: U.S. citizen's married sons/daughters. Subject to quota.
6. F4: Brothers/sisters of adult U.S. citizen (21 or older). Subject to quota.
Steps
In the big picture, a family-based immigration process consists of the following steps:
1. Filing I-130 Petition:
Your U.S. citizen or permanent resident family member must file a Form I-130, Petition for Alien Relative, along with proper supporting documents, with USCIS. It takes 4-8 months for USCIS to review the petition and make a decision (for petitions for siblings under F4, USCIS's review can take up to 24 months.
2. Waiting Time until Priority Date becomes Current:
Except for immediate relatives of U.S. citizens, each family-based preference has a set annual quota, totaling 226,000 per year. Depending on which preference you fall under, the number of cases filed before yours may be exceeding the quota for that preference, in which case you have to wait in line until it is your turn to proceed with the next step (immigrant visa application or I-485 application). Each case is given a "priority date" that determines its place in queue; your "priority date" is usually the date the I-130 petition is filed with USCIS.
When your priority date becomes "current", it means you can proceed with the next step. In order to check each preference's priority date that is soon becoming current, please visit the Department of State's monthly visa bulletin: http://travel.state.gov/content/visas/english/law-and-policy/bulletin.html. Please note that 4 countries (China mainland, India, Mexico, and Philippines) have their own respective quota and thus their own respective priority date. Also note that how fast/slow a priority date advances (or sometimes even retrogresses) can often change depending on how many cases are being filed at the time.
3. Immigrant Visa Application, or I-485 Application to Adjust Status:
Once your priority date becomes current, you can now:
----- Apply for an immigrant visa at a U.S. Embassy/Consulate (if you are outside the U.S.). Once the U.S. Embassy/Consulate issues an immigrant visa, the visa will be valid for 4-6 months only, during which time you must arrive in the U.S.; or
----- File a Form I-485, Application to Adjust Status, with USCIS (if you are in the U.S. in lawful status).
At the time your petitioner is filing an I-130 petition, if your priority date is current AND if you are in the U.S. in lawful status, you can file your I-485 application at the same time with the I-130 petition (concurrent filing).
As for concurrent filing by a U.S. citizen and his/her immediate relative, INA (Immigration and Nationality Act) allows the U.S. citizen to file an I-130 petition and the immediate relative to file and I-485 application, even if the immediate relative has fallen out of status or has been engaged in unauthorized employment. The immediate relative, however, must have entered the U.S. legally (unless s/he is eligible under 245(i) of INA, as explained under the sub-menu "245(i)").
Dependents
1. For F1, F2, F3, F4: Your dependents (spouse; unmarried children who are under 21 when the I-130 petition is approved or your priority date becomes current, whichever is later) can also file immigrant visa applications or I-485 applications to adjust status. If your child had already passed his/her 21st birthday when the I-130 petition is approved or your priority date becomes current (whichever is later), the Child Status Protection Act allows that his/her age be reduced, for immigrant visa or I-485 purposes, by the amount of time the I-130 petition was pending.
2. For Immediate Relatives of U.S. citizens: If you are an immediate relative of a U.S. citizen, your dependents cannot apply as your derivatives. Instead, the U.S. citizen petitioner has to file a separate I-130 petition for each of your depenedents from the beginning.
[Moving from one Preference to another]
After your U.S. citizen or permanent resident petitioner files an I-130 petition, it is not uncommon that you later become ineligible for that category (your initial category) because you get older or your marital status changes. As long as the changed circumstances still make you eligible for another category, the family-based immigration process can continue under the new category without going back to square one. However, if the changed circumstances do not make you eligible for ANY category, the I-130 petition will be revoked and the family-based immigration process is over; even if there comes a second change that would otherwise make you eligible again for one of the categories, the revoked I-130 petition does not revive. You have to start alll over again.