At various points in life, people leave one group or situation for another. When this happens, they proceed through three stages of a rite of passage, and this can be seen in a traditional Christian marriage ceremony.
First, there is a separation. This is most notable (and most traditional) when a bride's father walks her down the aisle and, after speaking a few words, a priest asks who will give the woman to be joined in marriage to the groom. The father usually supplies his consent at this point, and he kisses his daughter (effectively kissing her goodbye as part of their unified family). The giving away of the bride signifies that the bride's parents support the union and are thus transferring the care of their daughter to her partner.
The transition phase is the longest part of the wedding ceremony. During this time, the priest provides Scripture to guide the couple in their marriage. The couple may have songs performed that signify their unique relationship. The bride and groom say vows by joining hands, promising to leave their families, to forsake all others, to love through every difficulty and celebration in life, and to do it all until their final days. They then exchange rings, providing an external symbol of their union for the world.
In the final moments of the wedding ceremony, the incorporation phase is completed. The priest pronounces the couple married, and they leave the church together, reentering society as a newly married couple, no longer the single individuals they were before.