Getting around Nepal can be a demanding business. The unfeasible terrain and extreme weather conditions, plus a high level of disorganization, mean that trips seldom go exactly according to plan. On the other hand, Nepali cleverness will generally get you to your destination in the end. Though travel can be maddening, it also creates memorable moments by the score. The entire gamut of transportation options is available in Nepal, from hot-air balloons to elephants. One of the major considerations when using any form of public transportation is to avoid travelling during festival times, particularly major ones such as Dasain and Tihar (Diwali). Buses and planes are booked solid, and forget flying if you haven't booked well in prior.
There are no drive-yourself hire cars available in Nepal, but you can simply hire cars with drivers, or book a taxi. Buses are the chief mode of public transport in Nepal and in relative terms they're extremely cheap. Very often they're also exceedingly uncomfortable. They run pretty much everywhere and will stop for anyone. You can jump on local buses in any part of Nepal, but you'll find it much easier to get a seat if you catch a bus at its source rather than in mid-run. For longer-distance buses it's best to book a couple of days in prior.