The London bus service is still popular way of transport within London. However, if you're planning to visit London from Europe or some other destinations, you should devote attention to the other bus services that are in charge of that.
So with this being said, there are few bus categories that exist:
Modern double or single-decker busThe local bus network in London is one of the largest and most comprehensive in the world. Over 6,800 scheduled buses operate on over 700 different routes. Over the year this network carries over 1.8 billion passenger journeys. As for the fares, a single-journey adult bus ticket within London costs £2 with a few exceptions. Children under 16, and full-time education people under 18 travel for free. However, if you own an Oyster card, single-journey will cost you 90p. Another version of the bus ticket is one-day bus pass and it costs £3. Weekly ticket costs £13 and monthly £42.30.
You can find more info on this website.
The Night bus is a special version of the London red bus which operates during the nighttime. Every unit that is considered as a night bus has prefix N. It usually operates between midnight and 4:30 am.
Intercontinental busThere are few international bus companies and some of them are:
National Express (0870 580 8080) - all the info you need, like the costs, can be found on their website.
Megabus (0900 160 0900) - in other instance is the service where you pay per minute (60p), if you want to visit their website for the more info, here it is.
Green Line (0870 608 7261) - smaller company with main routes, you can visit their website here.
Eurolines (0870 514 3219) - connections with continental Europe, visit their website here.