US citizens have the right to bring in spouses, children, parents, siblings, and fiancé(e)s. This is called family based or family sponsored immigration.
Permanent Residents can sponsor spouses and eligible children.

Immediate Relatives

To count as an Immediate Relative (IR) one must be a spouse, an unmarried child under 21, or a father or mother of a US citizen.
The fiancé(e) of a US citizen becomes an IR as soon as he/she is married to the citizen.
Same-sex marriage also counts.

Family Members

For family-sponsored immigration the total number of visas that is available in a year ranges from 226,00 to 480,000.

Family-sponsored immigration is divided in four categories called “preferences”.
Each preference category is allocated a number of the available visas.

F-1      unmarried children over 21 of US citizens : 23,400
F-2A   spouses and unmarried children under 21 of US residents: 87,934
F-2B   unmarried children over 21 of US residents: 26,266*
F-3      married children of US citizens with eligible family members: 23,400
F-4      brothers/sisters of US citizens with eligible family member: 65,000

* More visas can be issued for F-2 preferences.

For some preferences the waiting times are long to very long. Especially for family members waiting to immigrate from Mexico, China, India, the Philippines. Not only because of the large numbers of family-sponsored immigrants-in-waiting, these countries are also much disadvantaged by the per country limit.
Waiting times can be found in the Visa Bulletin published monthly by the US Department of State.

To see waiting times go to the USCIS website.
Search Visa Bulletin, add current year and month, scroll to Family Preferences.