You can get to Cambodia by boats, planes and automobiles. The easy way is to fly and now with the addition of Air Asia, the Malaysian budget airlines, air travel is even better.
If you choose to travel overland to Cambodia you will get to enjoy more of the countryside.
The road from Vietnam to Phnom Penh is good and travel is relatively quick and uneventful.
The road from Poipet to Phnom Penh is also good as is the road from Phnom Penh to Siem Reap.
Other roads are usually in bad condition and travel times can vary.
If you are travelling from Trat in Thailand to Phnom Penh the boat to Sihanoukville and then bus to Phnom Penh is the best option.
The only problem with travelling overland is the border scams you may encounter. For more up to date information on border scams visit Tales of Asia.
If you want to take a boat from Vietnam that is also possible fast boats as well as cruise boats operate.