A common law marriage is when a couple claims to be married for all intents and purposes. However, they do not go through the process of being legally married (such as obtaining a marriage license and going through a legal wedding ceremony). Certain states recognize common law marriage as a legitimate form of marriage, and certain states do not. The reason why a couple may try to claim a common law marriage as opposed to just remaining an unmarried couple is because marriage comes with a number of legal benefits and rights.
Basically, a common law marriage is just when a couple who is living together starts declaring themselves as married, without going through the process of being legally married by a religious official, or by a justice of the peace, etc. In some states that recognize common law marriage, couples who live together in a romantic pairing may be considered common law married after a specific length of time. People who pursue common law marriages still have to meet some of the requirements of regular marriages. For example, both people must be at least 18 years old and both people must be sound of mind. Couples who have common law marriages can enjoy a number of legal benefits in certain states that they would otherwise could not.