Holy Water is water that has been exorcised and blessed mixed with salt that has been exorcised and blessed. It is mainly used by Catholics, but other liturgical denominations may use it as well, but in a less frequent ratio and with less emphasis.
Catholics bless and exorcise holy water frequently. When entering a church, one is obliged to dip their hands in the holy water and make the Sign of the Cross, blessing oneself. Holy water is considered to ward of demons, as a sacramental, and to forgive venial sins. It is sprinkled over the congregation during Mass.
Eastern Christians make holy water once a year at the Feast of Theophany. This holy water is to be drunk throughout the year for blessings. Holy water is also made when needed for baptism or exorcism, and at times will be blessed for the "Lesser blessing of waters." When an object is blessed, it is sprinkled thrice with holy water.
Anglicans and Episcopalians utilize holy water during baptism and upon entering churches. It is blessed beforehand, though it's not a commonplace practice.
Lutherans have taken to including a font full of water at the front of their church. It's not considered "holy" though, but merely as a ritual inherited from their Catholic forefathers.
Methodists consider the water used during baptism to be holy, mystically changed into holy water as the child is being baptized.
Some Evangelicals have taken to using holy water as a symbol to remember their baptism, without the actual water holding any power in and of itself.
Other forms of Holy Water
Holy water can also refer to special springs or places where the water was made holy by a supernatural event, such as the water at Lourdes.