Immediate Relatives of U.S. Citizens - Unlimited Categories
Immediate Relatives of U.S. Citizens (IR): The spouse, widow(er) and unmarried children under 21 of a U.S. citizen, and the parent of a U.S. citizen who is 21 or older. As there is no numerical limit on how many visas can be issued in these categories each year, qualified applicants don't have to spend time waiting for their priority date to become current (immigrant visa number to become available), once the petition is approved by the USCIS.

Spouse
A fraudulent marriage to a U.S. citizen, i.e., marriage just to get a green card is illegal. Only the couple can know the actual intention behind the marriage. There are many situations that can lead to suspicion of the USCIS such as a couple not living together, having just met, not knowing a common language to talk to each other and coming from extremely different backgrounds etc. There are many things, on the other hand, that would tend to prove a bona fide marriage exists such as children born of the marriage, staying in the same house, common finance, joint property ownership etc.

Marriage to the U.S. citizen must itself be valid, i.e., the husband and wife were each legally able to get married. For example, if either person had been previously married, a legally valid divorce must have since taken place. The actual ceremony must also, in the jurisdiction where it was performed, be legally recognized. Husband and wife also should be of legal age to get married.

The marriage must actually still exist; a former marriage to a U.S. citizen does not qualify to apply for a green card. While a marriage can remain in existence despite the physical separation of the couple, more serious arrangements, such as the execution of a legal separation agreement, do tend to generate substantial obstacles to immigration. It is also important to remember that if the marriage took place within the two years of getting permanent residence, it is done so on a "conditional" basis only. This means that if within two years after permanent resident status is obtained, the marriage terminates or is discovered to have been a "sham", the grant of permanent residence will be revoked. In every other respect, the "conditional" permanent resident is identical to all others. He or she may work, travel, and enjoy the full measure of benefits available to all lawful permanent residents.

IR-1 - Spouses of a U.S. citizen
CR-1 - Spouses of a U.S. citizen (conditional status)
IW-1 - Certain spouses of deceased U.S. citizens. A petition must be filed within two years of the death of the U.S. citizen
K-1 - Fiance(e) and minor children of a U.S. citizen
K-3 - Certain spouses and minor children of U.S. citizens

Children
Any child who is under 21 years old and is not married is covered under this category. As long as that child is the sponsoring U.S. citizen's child, it does not matter whether the child was born while the sponsor was married to the partner or not. A child born while the sponsoring parent is married to the partner, sponsoring parent can be either mother or father. A child born while the sponsoring parent is NOT married to the partner, if the sponsoring parent is mother, that will qualify. But if the sponsoring parent is the father it will qualify only if evidence exists that the father is financially supporting and/or living with the child and while in legal custody of the child and before the child's 18th birthday, has "legitimized" the child in the manner prescribed by law.

Step children, adopted children, and "eligible orphans" are also eligible for immigration in certain instances.

An immigrant visa may only be processed for a child if he or she has no claim to U.S. citizenship.

If you are filing for your spouse and if you have children immigrating with your spouse, and you are a U.S. citizen and going to get a K3/K4 visa for your spouse/children, you should submit separate visa petitions for those children as well. To do so, you must qualify as the child's parent or stepparent, and can only be their stepparent if your marriage took place before the child's 18th birthday. In such cases, filing separate petitions for children is not to get a K4 visa (which they can get based on your spouse's K3 visa), but eventually to apply for adjustment of status in the U.S.

Children of U.S. citizens need to have to their own petitions, that is, one petition per child.

IR-2 - Unmarried child under 21 years of age of a U.S. Citizen
CR-2 - Unmarried child under 21 years of age of a U.S. Citizen (conditional status)
IR-3 - Orphan adopted abroad by a U.S. Citizen
IR-4 - Orphan to be adopted in the U.S. by a U.S. citizen
IW-2 - Child of an IW-1
K-2 - Child of a K-1
K-4 - Child of a K-2

Parents
If the U.S. citizen child is 21 years of age or above, he/she can sponsor his/her parents for immigration. And the citizen must be the "child" of the parent seeking permanent residence, as described above.

A separate petition needs to be filed for each parent.

If parents of the U.S. citizen sponsor have minor children abroad, those children (your siblings) cannot be sponsored on the same petition. You will have to apply for your siblings under the F4 category. Alternatively, once your parents get a green card, they can apply for a green card for them under the F2B category.

IR-5 - Parent of U.S. citizen

Returning Residents
This category is actually NOT for the immediate relatives of U.S. citizens, but it is listed here because it has no annual quota.

Immigrants who lived in the United States previously as lawful permanent residents and are returning to live in the U.S. after a temporary visit of more than one year abroad.

All other categories other than described above are limited categories; they have numerical limits on how many people can immigrate each year under that category.

SB-1 - Lawful permanent residents who want to return to the U.S. after staying abroad more than one year or beyond the expiration of their re-entry permits