The Floo Network is a magical method of transportation in the Harry Potter novels, and can be used by witches and wizards at all levels of ability, because it does not require an act of magic from the traveler to work. A network of fireplaces across Britain are magically connected to form the Floo Network through a process which is never explained in the book, although it could probably be likened to the method used to connect telephone lines in the muggle world. When travel is desired, a witch or wizard uses a substance known as floo powder to activate the fireplace, and then steps in, states the destination, and travels through the network to another fireplace.
The name of the Floo Network is probably a pun on “flue,” for chimney. Floo powder turns the fire green, and appears to prevent it from burning the witch or wizard using it, as well. The amount of floo powder needed does not appear to be specific: Harry Potter takes “a handful” of it in Harry Potter and the Chamber of Secrets. Once the flames turn green, the traveler must loudly and clearly pronounce the desired destination, and then tuck elbows in for the journey, which whirls the wizard through the Floo Network, providing vague glimpses of other destinations, before finally ejecting the wizard on the other end. The procedure seems messy and somewhat unpleasant, with many travelers emerging covered in soot and sometimes with broken glasses, as well.
Showier wizards step into the fire and then toss the floo powder, probably using a Freezing Charm to render the flames of the fire harmless. Others use the Floo Network to talk to each other, transporting just a head, rather than an entire body. This rapid communication technique can be used in emergency situations in which the wizard's body must remain stationary, for various reasons. The Floo Network is also not without its flaws; if a wizard fails to clearly state the end destination, he or she will end up somewhere else entirely, and potentially in a dangerous situation.
A team of Floo Network Regulators work for the Floo Network Authority to manage the Floo Network, presumably also providing connection and disconnection services. For obvious reasons, muggle fireplaces are not included in the Floo Network, although Arthur Weasley does pull a favor during Harry Potter and the Goblet of Fire to have the Dursley's fireplace connected for the evening, so that he can pick up Harry Potter. For underage wizards who cannot yet apparate or perform magic outside of school, the Floo Network is the transportation method of choice.