The matrimonial covenant, by which a man and a woman establish between themselves a partnership of the whole of life and which is ordered by its nature to the good of the spouses and the procreation and education of offspring, has been raised by Christ the Lord to the dignity of a sacrament between the baptized.
[from the Catechism of the Catholic Church]
Marriage is both a vocation (calling by God) and a sacrament, which makes it doubly blessed and imparts special graces to aid couples.
Because of its covenantal nature, marriage is the only sacrament in which the participants confer it upon each other. In other words, the couple’s exchange of marital vows bestows the sacrament. The priest or deacon is still essential. He is the official witness who represents Christ and the Church.