http://en.wikipedia.org/wiki/Common-law_marriageCommon-law marriage (or common law marriage), sometimes called de facto marriage, informal marriage or marriage by habit and repute, is a form of interpersonal status which is legally recognized in some jurisdictions as a marriage even though no legally recognized marriage ceremony is performed or civil marriage contract is entered into. A common law marriage is legally binding in some jurisdictions but has no meaning in others. In some jurisdictions without true common law marriages (e.g. Hungary), the term "common law marriage" is used as a synonym for non-marital relationships such as domestic partnership or reciprocal beneficiaries relationship.
Common-law marriage can still be contracted in 11 states and the District of Columbia, can no longer be contracted in 26 states, and was never permitted in 13 states. The requirements for a common-law marriage to be validly contracted differ from state to state. Nevertheless, all states — including those that have abolished the contract of common-law marriage within their boundaries — recognize common-law marriages lawfully contracted in those jurisdictions that still permit it.
There is no such thing as "common-law divorce" — that is, you can't get out of a common-law marriage as easily as you can get into one. Only the contract of the marriage is irregular; everything else about the marriage is perfectly regular. People who marry per the old common law tradition must petition the appropriate court in their state for a dissolution of marriage.