Express buses are the most common buses in Cambodia. They generally have 45-55 seats in rows of four, with two on each side of the aisle. The seats can often be a little worn out but will recline partially and be comfortable enough for any day trip in Cambodia. Taller passengers should be warned that the legroom can be slightly cramped on many older buses. You can usually bring as much luggage with you as you would when flying, and larger pieces can be stored below the bus.
Local Cambodians frequently travel on Express buses, especially during holiday and festival periods, so that they can sell out a few days before departure. All Express buses should have A/C (which may be too strong) and make rest stops every few hours at prearranged times. The food at these rest stops often leaves much to be desired, but plenty of prepackaged snacks and drinks will keep you going.
Traveling in Cambodia by van is often faster than traveling by larger buses, but what you make up for in time, you lose comfort. Vans likely seat 12 or 15 passengers and it is rare for one not to be full once you depart. Luggage space is limited, and at least one piece of whatever you bring will probably end up on your lap or at your feet.
The drivers on all buses, but especially vans, may have a style of driving that would, by the standards of your home country, be best described as ‘crazy,’ ‘insane,’ or ‘suicidal.’ But rest assured that they are none of the above and behave normally for this part of the world.