Kiddushin, from a root meaning to set apart or sanctify, is effected when the groom gives the bride an object of value, in nearly all traditions today a plain ring without stones, in the presence of two qualified witnesses, while reciting a declaration that with this object she is set apart for him according to the law of Moses and Israel. The bride's acceptance of the object completes the act. From that moment she is legally married to him in the sense that any other union would require a divorce to dissolve, but the couple do not yet live together as husband and wife.
Between kiddushin and nissuin, tradition calls for the reading aloud of the ketubah, the marriage contract, which sets out the husband's financial obligations to the wife, including provision in the event of divorce or his death. In the classical period the two stages were commonly separated by as much as a year, giving the groom time to prepare a household, but the delay created hardship and risk, particularly for a bride left legally bound but not yet supported, and by the Middle Ages the two ceremonies had been merged into one occasion to remove the gap altogether.
The whole arrangement is treated in Jewish law as a matter of contract law given religious form, which is why the tractate of the Mishnah and Talmud devoted to it, Kiddushin, sits within the legal rather than the purely liturgical portion of the Talmud.