US citizens have the right to bring in spouses (husbands/wives), unmarried minor children, parents (fathers/mothers).
These count as an Immediate Relative (IR) of the US citizen.

There are no quotas for IR’s, which means no waiting time for a visa.

For immigration purposes a minor is a child under 21 years of age.

To sponsor a parent the US citizen must be an adult (21 or over).

The fiancé(e) of a US citizen becomes an IR as soon as he/she is married.
Same-sex marriage also counts.