In the Harry Potter books, it's what wizard use to get from place to place. They throw some Floo Powder into their fireplace and say the name of the location in which they wish to travel. This is not the only form of transportation though, as wizards also use Apparition, Portkeys, and Broomsticks, flying Ford Anglia cars, flying motorcycles, and Thestrals.
Let's use the Floo Network to get to Diagon Ally today. I'm too lazy to apparate.