Floo Network
The Floo Network joins fireplaces so that witches and wizards can travel by fire from one hearth to another, though Lupin warns that it can be watched.
Mr Weasley has the Dursleys' fireplace hooked up to it for a single afternoon to collect Harry, with help from a well-placed friend on the Floo Regulation Panel, though strictly speaking Muggle fireplaces are not supposed to be connected. The Dursleys have boarded theirs up behind an electric fire, so the Weasleys get stuck behind the boards until Mr Weasley blasts their way out. At the Ministry, the Floo Network Authority sits within the Department of Magical Transportation.
How far into Harry Potter have you read?
Set your place and this page rewrites itself to exactly that point. Nothing ahead of you will show.