Self-explanatory really: a bus you can catch at night. I'm noding about it because it's an essential part of London nightlife.
Unlike London Tube Etiquette, conventions on night buses are more relaxed. Even though they're more likely to be asleep, drunk and so on, people are generally more willing to move up and give you a seat.
It's also one of the few places where different subcultures mix; most people are going home from clubs. Nu metal kids in Slipknot tshirts, goths and ravers, and even trendies, will been seen talking to each other. It's not unknown for passengers upstairs to be rowdy, sing songs loudly and even smoke pot.
Night buses are much cheaper than cabs and are therefore popular with people who live quite a way out of town. For example, a mini cab from Oxford Street to Edgware might cost around £20, but the same journey by bus costs £1.50. It's not unusual to see hundreds of people freezing to death waiting for one in the middle of December. However, it's not unusual for the bus to be so full that the driver won't let any more people on. This is most likely to happen in Camden Town between 2 and 3am.
It can be a bit of a bumpy ride, so I'd advise against drinking a McDonalds milkshake before boarding, or you might wish you hadn't.