What are the differences between civil unions and marriages?

The biggest difference is that marriage has been a recognized legal institution for hundreds of years in the United States (and around the world), while civil unions are a relatively recent phenomenon. Civil unions provide for benefits in the state that recognizes such unions, but those benefits may not be recognized elsewhere. Perhaps the primary difference is that many advocating for equal treatment for gays and lesbians assert that civil unions accord a second class legal status than marriage. For example, the group Gay and Lesbian Advocates and Defenders (GLAD) writes that marriages 276 are far more likely to be respected by others than newly minted civil unions.