The Ketubah is the original pre-nuptial contract. Technically a marriage without a ketubah is considered mere concubinage. It is a contract between the husband and the wife that specifies the husband’s basic duties to the wife, including the amount that he must have “set aside” for her should he die or should they divorce. Like any contract it must be signed. The groom’s signature makes the proposal official. The bride accepts the proposal by signing. And there are witnesses who must sign as well. Their signature is to ensure that the groom is acting in good faith, as well as to finalize the contract.
“The ketubah restates the fundamental conditions that are imposed by the Torah upon the husband, such as providing his wife with food, clothing, and conjugal rights, which are inseparable from marriage. It includes the husband's guarantees to pay a certain sum in the event of divorce, and inheritance rights obligatory upon his heirs in case he dies before his wife.”
The tradition is over two thousand years old. Much of the language has been codified and the sums in the text are often what amounts to token or historic values, but never the less it is a written promise; a promise that is displayed in the home as a cherished artwork. A couple is instructed to always know where the ketubah is located. If they cannot locate it, they are supposed to have a replacement document generated by qualified rabbi.
In theory it serves as a protection for the wife. Since Jewish tradition does allow divorce and traditionally inheritance passes to sons, not wives, it is some small assurance for her financial well being if the unexpected or unfortunate happens. It is supposed to define a woman’s rights and man’s duties within a marriage. It is supposed to make a man consider her financial well being before seeking a divorce. Ideally all of this would be true.
As I have learned more I find that they are traditionally written in Aramaic – the legal language of Talmudic Law, but can be found in Hebrew or modern languages such as English. The text must be okayed by the Rabbi performing the ceremony, and there are several different modifications available. Apparently there are even non-denominational Ketubah’s for non-Jews.