Taking the bus is currently the most convenient way to travel from one city to the next. Large, air-conditioned buses will take you almost anywhere you want to go, while offering a certain amount of luxury. They connect Phnom Penh to Siem Reap, to Battambang, and to many other towns.
You can also reach surrounding towns and villages of Phnom Penh by using local bus services. Many of the staff even speak English and can help you find the right connection. Keep in mind though, that these are not as luxurious are the air-conditioned, long distance buses mentioned above.
Provincial routes are usually serviced by minibuses which are mostly used by the local population. While they are cheap, the journey can be very crowded and uncomfortable. The drivers are not the most cautious you can find and if you are prone to travel sickness, you should avoid this means of transportation altogether.