What Constitues as a Marriage?  

What makes something a true marriage these days? I know that many people take it as a church wedding, one with a certificate and made under the approval of God. Still, I know that in biblical days, marriage was often less under restrictions. I have heard that when Joseph and Mary became engaged, the simple act of her moving in with him was considered all right because in many people's eyes they were already married.

Today it is far more under strict rules and guidelines. Does this truly make it a marriage? Is it more legal and political these days then in the past? If two people give their own vows and believe themselves to be married in God's eyes wouldn't this be enough?