If you are a U.S. citizen and 21 years old or older, you may petition for a green card for your parents (if they are in the U.S.; if they are abroad then they get a immigrant visa to travel to the U.S.) To qualify, the parent must meet the legal definition under U.S. immigration law, and may include a stepparent, an adoptive parent, and a parent of a child born out of wedlock (though the applicant may have to establish the “parent-child relationship” by blood/DNA tests, evidence of cohabitation, support and communication.)
For a stepparent to qualify, your step-parent must have married your birth parent before your 18th birthday.
For an adoptive parent to qualify, the adoption must have taken place before your 16th birthday. Furthermore, your adoptive parent must have had legal custody of you for at least two years, either before or after the adoption and you must have resided with the adoptive parent for at least two years before or after the adoption.
It should be noted that the father-in-law and mother-in-law of a U.S. citizen are not “parents” of a U.S. citizen for immigration purposes.