The range of road transport is extensive. On sealed roads, the large air-conditioned buses and speedy express minivans are the most popular choices. Elsewhere in the country, a shared taxi or local minibus is the way to go.
All major cities are now well-linked by bus to Phnom Penh along sealed roads, but if you're travelling from one end of the country to the other you may have to change buses in Phnom Penh or another hub.
While it doesn't cover all bus companies, bookmebus (www.bookmebus.com) is a reliable bus-ticket booking site, including for more obscure routes (Ban Lung to Siem Reap, anyone?) and cross-border trips.
Buses are reasonably safe but accidents can happen on Cambodia's dicey roads, and there have been several big accidents involving buses or express minivans where tourists were killed.
Express minivans, which usually take the form of modern Ford Transits or Toyota Hiaces, operate a one seat/one passenger policy and are reasonably comfortable, but they are sometimes driven by maniacs, so check the reviews.
Older local minibuses serve most provincial routes but are not widely used by Western visitors. They are very cheap but painfully slow and often uncomfortably overcrowded, with people spilling out the back and kids vomiting everywhere; only really consider them if there is no alternative.