Holy water, which is ordinary water that has been blessed by a priest or deacon, is one of the most common sacramentals in Catholic life. Traditionally, Catholic churches have a small font (a bowl or dish) containing holy water just inside every doorway into the church, either mounted on the wall or free-standing. We bless ourselves with this water (that is, we dip our fingers in the font and then make the sign of the cross) as we enter and leave the church, as a reminder of our baptism.
Some Catholics have similar fonts in their own home; when I was growing up, I had one mounted just inside the door of my bedroom. There is a holy water dispenser available somewhere in most churches so that people can bring the water home to fill these fonts.
Holy water can also be used to bless other objects by sprinkling them with it while praying over them; the book Catholic Household Blessings and Prayers includes a number of such prayers suitable for family use, such as blessing an Advent wreath or a Christmas tree.