Routemasters are the buses that don't have actual doors for you to get on through, just an open bit at the back where you can hop on and off wherever you like -- at an official bus stop, or while the bus is randomly stopped in traffic.
Routemasters are also different from other London buses in that they have both a driver and a conductor. The driver is sealed off in the cab and the conductor floats freely around the bus, checking and selling tickets every so often. If the conductor considers the bus to be full, they'll most likely be on the open bit at the back stopping anyone else from getting on. Different conductors seem to have different ideas about what "full" means, but there's no point in arguing with them; if you piss them off enough then the driver will just stop the bus and nobody's going anywhere.
The thing that confused me for ages about routemasters is whether you're meant to show your ticket or pass to the conductor when you board. You're not. If they want to see it, they'll come round and ask once you've sat down.
The conductor is supposed to know whether there are seats upstairs or not, so you can generally ask them when you get on, to save a fruitless climb upstairs, but some of them can't be arsed to keep track. Always worth asking, though.
- http://www.london-rip.com/routemasterrip.html (for a critical view of the Routemaster)