Larger towns and cities will have bus stations. Buses may also serve their companies' offices, which may be more convenient than the bus station: this is particularly true in Siem Reap. Mekong Express has the best reputation for comfort and speed and consequently charges a premium. Sorya (formerly Ho Wah Genting) and GST offer a slightly cheaper no-frills service. Capitol Tours which offers basic cheap runs between its centrally located offices, making for convenient city centre-to-city-centre travel as well as daily service to Bangkok and Vietnam. Ramshackled peasant mover Paramount Angkor Transport is great for accessing more remote places but low on comfort and safety.
Generally bus travel is cheap, with journeys from Phnom Penh to Siem Reap or Sihanoukville costing around US$5. Bring along something warm if you don't like freezing air-conditioning and earplugs if you don't like Khmer karaoke. There are a few night-time services but most buses leave in the morning and the last ones leave in the afternoon.