The traditional ketubah is a legal instrument and reads like one. It records a sum owed to the wife if the marriage ends, conventionally two hundred zuz for a first marriage and one hundred for a subsequent one, together with whatever additional amount the husband undertakes. It records the property she brings and on what terms it is held. It obliges him to provide food, clothing and conjugal relations, following the verse in Exodus, and the rabbinic elaboration adds medical care, burial, maintenance of her from his estate after his death, redemption from captivity if she is taken, and provision for her sons to inherit her portion.
It is written in Aramaic, in a set formula, and signed by two witnesses who must be qualified and unrelated. It is given to the wife and is hers to keep, and traditional law holds that a couple may not live together without one, so a lost ketubah is replaced before anything else happens.
The Talmud credits Simeon ben Shetach with the decisive reform: making the obligation a lien on the husband's whole estate rather than a sum deposited with the wife's family. The stated purpose is that divorcing should not be a light matter, since a man ending a marriage has to find the money. A document that reads coldly is doing protective work, and the rabbinic sources say so openly.