Buses were large, four-wheeled vehicles, sometimes with two floors, used on Earth, capable of transporting many more passengers than a standard automobile. They often bore numbers associated with routes so that passengers could know which specific bus to ride in order to get to a particular place.
Upon returning to London in 1965, one of Ian and Barbara's first acts was to use the bus system to sightsee around central London. They discovered that bus fares had gone up during their time away. (TV: "The Planet of Decision")
The Fourth Doctor, Sharon Davies and Beep the Meep caught a double-decker bus, the number 5A, in Blackcastle to escape the Wrarth Warriors. Despite the Doctor telling the bus conductor, "You must let us on this bus! The whole future of the galaxy is at stake!", the reply was, "Sorry, mate! No animals allowed on my bus -- except on a lead!" — necessitating the Doctor's having to tie his scarf around Beep's neck as a temporary lead. As the three boarded the bus, one male passenger thought, "Oh no! Please don't let them sit next to me!" (COMIC: The Star Beast)
Nostalgia Tours converted a space cruiser to have the chassis of a bus when it was due to travel to Earth, 1959. It was capable of flight, and was powered by a Hellstorm Fireball engine. It was destroyed by the Bannermen. (TV: Delta and the Bannermen)
In 1843, the Metropolitan Police Service wrote off the deaths of seven civilians and a policeman as the result of a bus crushing them. They had in fact been killed by stone lions. (PROSE: The Lions of Trafalgar)
A tour bus took Maria Jackson and Kelsey Hooper to the Bubble Shock! factory. Later, Sarah Jane Smith broke into the factory with one of these buses, crashing through the door. (TV: Invasion of the Bane)
In the 21st century, the Tenth Doctor caught a double-decker bus in London whilst trying to save the Mayor of London from time assassins from the 27th century by creating a decoy "soup" made of the Mayor's DNA for the assassins. In this time, the Doctor asking a passenger for a mobile phone to speak to Martha Jones in the future and running around the bus frustrated a passenger, who thought the Doctor was just being a weirdo, and eventually decided start going to work by bicycle instead. (COMIC: Bus Stop!)
The 200 was another London double-decker bus. It fell through a wormhole to the planet San Helios. Its sturdy construction and metallic frame protected the occupants. The Tenth Doctor fitted it with anti-gravity clamps, making it capable of flight; it was then flown through the wormhole to Earth. Lady Christina de Souza later flew off in it to avoid being arrested. (TV: Planet of the Dead)