The 6-2 bus: In a 4-4-2, full-backs tuck in and wide midfielders are responsible for marking the opposition wingers. 6 central players can disrupt the opposition's passing game, and even if the wide players are beaten one-on-one, the central mass of players should be able to sweep up any crosses into the area. However flat lines of defence can be breached if one player strays from the line and plays opposition attackers onside (e.g. Pique goal vs Inter, CL Semi 2nd leg 09)
The 5-3 bus: Slightly rarer due to the decline in 3-man defences in the modern game. A spare centre-back allows for close marking of any free players, and 3 narrow midfielders help block the spaces in front of the defence. The wide areas are conceded in the expectation that any crosses into the box will be cleared by the mass of players. However this can be exploited if the wide player decides to shoot instead (e.g. Maicon goal vs North Korea, WC 10 Group Stage)
The last minute bus: This is the sort of bus you employ when you're leading 1-0 in the Champions League final in stoppage time. While its numbers are strong, it lacks in organisation and can self-destruct through mass panic.