The bus network is quite recent in Cambodia and developing. It is the only public transportation and also the cheapest way to explore the city. The bus fare is 1500 riels (which is less than US$ 0.25). You pay every time you hop in the bus (therefore, every time you change bus). 
For the moment there are only 3 lines. From our airport you can catch bus n°3. It will take you to the city by passing through Russian boulevard, 7 Makara bridge, Kampuchea Krom boulevard, straight to Phasar Thmey Market, then south to Norodom boulevard, and ends at the Night Market on street 106.

The line operates from 5:00am to 8:30pm. Open line 3 map. For more information, visit the City Bus facebook page.