Baptism is a term used by Christians to describe a ritual washing initially connected in the Gospels with John the Baptist and later commanded by Jesus of his followers. The most commonly cited reference for the command is the "Great Commission," found in the Gospel of Matthew, chapter 28, verses 18-20. It is typically viewed as a rite of passage into the Church, after which the newly baptized person is considered to be a Christian.
Those Christians who consider baptism to be a sacrament typically use sanctified water, and practice 'sprinkling' as a form of baptism, but usually use a baptismal font. Those Christians who consider baptism to be an ordinance typically also translate the Greek word 'baptismo' literally as 'dunking' or 'immersion', and do not ritually sanctify the water prior to baptism. Those who practice dunking or immersion, may do so indoors in a baptismal pool, or outdoors in a creek or river.
The liturgy of baptism in the Catholic and Eastern Orthodox tradition makes clear reference to baptism as a symbolic burial and resurrection, and draws parallels to the experience of Noah and the passage of the Israelites through the Red Sea divided by Moses. Thus baptism is symbolically not only cleansing, but also dying and rising again with Christ. Those who do not have a liturgy of baptism also use the same parallels in scripture.
The Catholic Church does not require any of the standard ritual to be performed. "In case of necessity any person, even someone not baptized, can baptize, if he has the required intention." The Church prefers the the formula "N., I baptize you in the name of the Father, and of the Son, and of the Holy Spirit," be said and the sign of the cross be made over the recipient, but even that can be omitted. The Church "has always held the firm conviction that those who suffer death of the sake of the faith without having received Baptism are baptized by their death for and with Christ. This Baptism of blood, like the desire for Baptism, brings about the fruits of Baptism without being a sacrament." (All quotations are from the Catechism of the Catholic Church and all emphasis is in the original.)
Baptism has strong links to Judaism, in which ritual washing to remove ritual impurity is an important element. [We need some reference here.] Orthodox Jewish communities are outfitted with a Mikvah or ritual bath.