The Floo Network is a mode of transportation in the wizarding world in which a witch or wizard goes from one place to another by means of Floo powder and a fireplace. Many fireplaces are connected in the Floo Network, and the traveller need only speak the desired fireplace they wish to arrive at very clearly as they stand in the emerald green flames.
The network governed by the Floo Network Authority, a section of the Department of Magical Transportation of the Ministry of Magic. This department is located on Level Six of the Ministry. The Floo Network Authority controls which fireplaces are connected to the Floo Network, which requires specialized magic. Travelling by way of the Floo Network can be done without specialised magic, and can be preferable for wizards too young to Apparate or those without access to other forms of transportation, such as broomsticks.
This network connects all wizarding fireplaces that utilise Floo powder as a mode of transportation. Muggle homes are not connected to the Floo Network unless special, temporary arrangements are made, which do not require that the Authority visit the location. The connection can be made from the Ministry of Magic.
To access the network, one must toss a handful of Floo Powder into a fireplace, walk into the heatless, emerald green fire, and declare the desired destination. It is apparently also important for the traveller to keep their elbows close to their body. During the trip, the traveller will notice other fireplaces and hearths zooming past, but it is not possible for the traveller to see these locations clearly as the speed of travel is too great.
One may also use the Floo Network to speak to someone in another location by merely putting their head through the green fire, as Sirius Black did to talk to Harry in latter's fourth and fifth years at Hogwarts. The Floo Network can also be used to transport inanimate objects. For example: In 1994, Molly Weasley uses the fire tongs to put a piece of toast into Amos Diggory's mouth while he is talking to Arthur Weasley via the Floo Network after the Quidditch World Cup incident.
In 1994, the Floo Network was temporarily connected to the Dursley's, as a favour to Arthur Weasley, so that he could transport Harry back to the burrow in preparation of the upcoming trip to the 1994 Quidditch World Cup that Harry would be attending with the Weasleys and Hermione.
In 1996, Harry used the Floo Network connection in Dolores Umbridge's office to contact Sirius Black at 12 Grimmauld Place, as her fireplace was one of the only ones not being monitored. Harry attempted to contact Sirius a second time on 17 June, but was caught by Umbridge while speaking to Kreacher.
At the end of the Christmas holidays in Harry's sixth year at Hogwarts, numerous students including Harry, Hermione and the Weasleys, used the Floo Network to travel back to Hogwarts.
In 1997, when the Ministry was overthrown by Lord Voldemort, only the homes and offices of senior Ministry officials were connected to the Network.