Cambodia is a very poor country, and as a consequence the public transport there is very limited. This will either force you to remain patient during your stay in Cambodia or find another way of travelling. In any case, one thing is sure: you are going to see a lot of the country!
Let's start from the beginning: there are no trains, no metro and almost no public transport at all in Cambodia. You will see upon your arrival in the country that the tuk-tuk is the main method of transport there.
In fact, as soon as you enter the larger cities such as Phnom Penh and Siem Reap, you will find a great deal of tuk-tuks ready to take you wherever you want for just a few dollars. Feel free to negotiate the price, as they can sometimes soar! You can usually take off one or two dollars per trip to arrive at a fair price. In the two cities the tuk-tuks and motor-taxis (also called motodops) are everywhere, at the airport at 5am until the last bars closing at night.
There are some buses in Phnom Penh, but they are rarely used by travellers. They are not very practical or cheap to use. In fact, it is probably best to spend the whole day with the same tuk-tuk driver, who will give you one price of around 10-15 dollars per day and will wait for you at every stop. Some of them also double up as tour guides!
To get between the different cities in Cambodia without difficulty during your stay, make sure you give yourself time to sleep, daydream and admire the landscapes.
There are many bus companies which can help you get from one city to another. However, the roads are in a very poor condition, especially the one that links Siem Reap and Phnom Penh: the roadworks happening there don't look like they're going to finish any time soon! Therefore it would take you about 7 hours to travel the 300 kilometres between the two cities. It's the same between Phnom Penh and Battambang If you are in Battambang and want to travel to the coast, for example, it would take you between 4 and 5 hours to get to Sihanoukville.
One thing to bear in mind: the local bus companies, such as Sorya Bus and Capitol Tours, often stop en route to their destination to drop off supplies or pick up other passengers, making the journey a little bit longer but also more local! The Giant Ibis bus company is the most luxurious in the country, and also the quickest.
You could save time by taking a flight: it takes 30 minutes to get from Siem Reap to Phnom Penh, or 45 minutes to Sihanoukville, by plane. It's also very practical if you're struggling for time.