Introduction to Masechet Kiddushin. Kiddushin (engagement) deals with the act of a husband taking a wife, making them husband and wife forever. It is an act of sanctification, a ceremony, a transaction, where a husband, for lack of a better word, acquires a wife. The Rambam tells us that before Matan Torah (Giving of the Torah), a man would meet a woman and they would just live together. They would decide that they were going to live together, and that would be the husband-wife relationship. When they wanted to end that arrangement, they would just end it. There was no get (divorce document) to end it as well.
But the Torah tells us that in Halacha (Jewish law), before a man marries a woman, he has to take her. He has to do something that will form a bond between them. As the Pasuk (verse) says, “When a man takes a woman.” The word Kiddushin establishes that the woman is Hekdesh. She's sanctified. That means she's only available and allowed to have a relationship with the man who sanctified her—the husband at that point in time. If she violated that, she would be violating the marital bond that they share.
There are two stages in a Jewish marriage. The first one is Kiddushin or Erusin (engagement); we use those words interchangeably. The second one is Nisuin. Kiddushin, or Erusin, which we'll call engagement, is Halachic (according to Jewish law) engagement. There's no perfect English word that goes with it, but it makes a husband and wife legally married. A woman is not allowed to have a relationship with anybody. At that point in time, if she did have relations with another man, she would be chayevet mitah, death penalty for adultery. She would need a get to end that relationship as well.
But at the same time, the husband and wife are only engaged. Kiddushin or Erusin has happened, and therefore they're not allowed to live together at that point in time. They're allowed to live together once Nisuin happens. Nisuin, which is the final step in the relationship between the husband and wife, is where he finally gets engaged and finally takes her as a wife; this is done through Chuppah (wedding canopy). Chuppah is where the woman formally enters Yichud with the husband. She enters a secluded area with him, or she goes under the Chuppah wedding canopy with him. At that point in time, they're fully married. They're allowed to have relations. A relationship between them could take place. At that point in time, they're fully considered to be a married couple. The husband, for example, would be obligated to support her.
In earlier times, there would be a stretch between the Kiddushin stage of marriage and the Nisuin. The bride would get engaged and then go live back at her father's house for an extended period, until the Nisuin happened. But nowadays, they happen right after each other.
Hachamim learn from the Pasuk, “When a man takes a woman,” that Kiddushin has to be done by a man. The woman accepts the Kiddushin. The husband proposes the Kiddushin. It happens in one of three ways. Kesef: The man hands over an object of value to the woman. He can give her a ring or a coin—anything of value—and he hands it over to her. By giving her a Shtar, a Kiddushin document. Or by bi'ah, by having relations with her for the purposes of Kiddushin. This was later outlawed by the Hachamim (Sages) because it wasn't considered to be a modest way of getting married.
Kiddushin is valid only if it's done in front of two witnesses, and the man has to make it clear that he's doing an act of Kiddushin. He can't hand her something, like a coin, if it isn't understood that it's an act of Kiddushin. He has to hand her that coin and say, “You are now sanctified unto me with this coin.” The same Halachot (laws) would apply to Kiddushin as to other legal acts as well. You could do it through a Shaliach (agent), just like you could do other legal acts through a Shaliach. You could do that as well.
A man could appoint a Shaliach to get a woman engaged to him, and a woman could appoint a Shaliach to accept Kiddushin for her. A man could give money to a Shaliach or give a Shtar to a Shaliach and tell him, “I want you to take this and get engaged to this woman for me. Have her engaged to me.” A woman could appoint a Shaliach to accept Kiddushin from a specific man for her. She would tell the Shaliach, “I want you to go and accept the coin, or accept something or a document on my behalf.”
There could also be a Tenai (condition) when it comes to Kiddushin, as in all other legal situations. A man could say, “You are engaged to me under the condition that I give you 200 Zuz (monetary unit).” That would be a valid Kiddushin if the Tenai, the condition, is fulfilled. But if not, then it would not be a valid Kiddushin.
There are three stages in a woman's maturity. She's first a Ketanah, a minor, from the time she's born until she's 12 years old. Then she hits puberty; she's grown two hairs. At that point in time, she becomes a Na'arah (young woman). Once she's a Na'arah, that's at 12 and puberty. Once she hits Na'arah, she's a Na'arah for six months. Then she becomes a Bogeret. A Bogeret is a full adult.
Kiddushin requires Da'at. It requires consent. Therefore, a minor is not allowed to give legal consent to authorize a marriage. So a man cannot do Kiddushin with a minor. But the Torah gave her father the authority to do Kiddushin on behalf of his daughter. He could also accept Kiddushin for his daughter who's a Na'arah. But all of this was eventually forbidden by the Hachamim. On a de-rabbanan (rabbinic) level, you're not allowed to marry your daughter off while she's a minor. You have to wait until she's mature enough. She could make her own decision and see if she fits with the man in question. Once she's a Bogeret, though, once she's an adult, she's allowed to accept Kiddushin on her own behalf. And that's how the Halacha is.