What is marriage?
Traditionally marriage is a civil contract that exists between two people of the opposite sex. In the past, marriage was considered to be between people of opposite genders. However, the times are changing, and same sex marriages are now being considered in some states. Only in very few states are same sex marriages actually legal. To be "capable of contracting", both persons typically must be of age (18), or have consent from a parent. Different states have different statutes on the "legal age" for marrying. The individuals must be unmarried and not divorced within the past six months.
What is common law marriage?
Common law marriage is only allowed in a small number of states. In a common law marriage, the individuals do not have a marriage ceremony and never obtained a marriage license to get married. Basically, a common law marriage requires an arrangement between the people (of opposite sexes) to act as husband and wife. This agreement can either be made by conduct or by words. The people involved must represent themselves (act in a way to appear to be married) to others as being married. For example, if they use the same name, call each other husband and wife, have children together, open joint bank accounts, and live at the same residence, all of these "indicia" of being married may be sufficient.