According to Torah law, marriage is a two-step process. The first stage is called kiddushin, loosely translated as "betrothal," and the second step is known as nisu'in, the finalization of the nuptials. Both kiddushin and nisu'in are accomplished successively beneath the chupah: the kiddushin is effected when the groom gives the bride the wedding band, and the nisu'in through "chupah" -- the husband uniting with the wife under one roof for the sake of marriage.
Kiddushin means "sanctification" -- signifying the uniqueness of the Jewish marriage where G‑d Himself dwells in the home and the relationship is elevated to a new level of holiness.
The mitzvah of marriage is performed over a cup of wine. "Wine cheers man's heart," and there is no mitzvah which is more joyously celebrated than a wedding. The rabbi holds a cup of wine and recites the blessing over the wine and then the betrothal blessing, which thanks G‑d for sanctifying us with the mitzvah of betrothal before consummating marriage. The groom and bride are given to sip from the cup.
The groom then places the wedding band on the bride's finger. While putting the ring on her finger, the groom says: "With this ring, you are consecrated to me according to the law of Moses and Israel." The betrothal must be witnessed by kosher witnesses in order to be valid.
For more detailed coverage of this subject, see The Betrothal in our Wedding Handbook.