|Muggles' Guide to Harry Potter - Magic|
|Features||Purple triple-decker bus|
|First Appearance||Harry Potter and the Prisoner of Azkaban|
The Knight Bus is a means of transportation for the stranded wizard; it will carry you from anywhere to anywhere (in England anyway) if you have the fare.
The Knight Bus is summoned by raising your wand arm. It is fitted inside with four-poster beds, and you can get a toothbrush or a mug of hot chocolate if you choose; though it's probably best to skip the latter, as the ride is rough enough to cause you to spill your mug.
In Harry Potter and the Order of the Phoenix, the Knight Bus appears in its daytime configuration. The four-poster beds are replaced by ordinary chairs, which like the beds are not anchored, so they slide around the inside of the bus with every swerve.
One side note: The Knight Bus in the movie of Harry Potter and the Prisoner of Azkaban contains, as decoration, a number of shrunken heads which talk. The author is on record as saying that, while she didn't think that detail up, she rather wishes she had. 
- Interview on the extended DVD set