A marriage and a civil union serve the same purpose: to unite two people in a recognized bond and provide legal protection to each of them. But civil unions, which had been the only commitment option for gay and lesbian couples until recently, have become much less popular. In 2015, the United States Supreme Court, in a 5-4 ruling, declared marriage between two people of the same sex legal in all 50 states.
"No union is more profound than marriage for it embodies the highest ideals of love, fidelity, devotion, sacrifice, and family," wrote Justice Anthony Kennedy in the majority opinion of the court. He noted "as some of the petitioners in these cases demonstrate, marriage embodies a love that may endure even past death. They ask for equal dignity in the eyes of the law. The Constitution grants them that right." The landmark decision gave married gay and lesbian couples across the entire U.S. the same federal and state benefits previously available only to straight couples.
Civil unions still exist but only in a handful of states, including New Jersey and Hawaii. Other states like Illinois and Vermont allow couples who were joined by civil unions prior to the 2015 ruling to convert them to marriages. So, although civil unions offer some of the same benefits as a marriage, they're only recognized at the state level—not federally.
Some of the federal benefits of marriage that civil unions don't offer include being legally recognized by all 50 states and every nation around the world. Spouses have the right to make medical decisions for one another; if you're young and healthy, this may not seem important to you right now but could be a major factor when you get older. Married couples have the right to file a joint tax return, which may offer significant tax breaks. Also, if one member of the couple is foreign-born, the American spouse is permitted to sponsor their partner for citizenship. There are other federal benefits too, such as those involving Social Security benefits and life insurance.