By Air: Siem Reap International Airport is the second largest Airport in Cambodia. A lot of major airlines operate their flights in and out from here.
By Bus: A lot of major companies operate from Bangkok to Siem Reap. But these direct buses are not very safe- the ones that operate directly from Bangkok. Although, they have various measures to avoid theft; travelling from any place in Cambodia to Siem Reap is fairly easy. You can travel in from Ho Chi Minh Bus stop to if you are looking at coming in from Vietnam. There are buses operating from Laos and Thailand as well.
By Boat: You can get in from Phnom Penh or Battambang. Even though the experience is beautiful, it is very time consuming.
By Train: Sisophon is the nearest station to Siem Reap. You can take a train from Phnom Penh which heads to Poipet and get down on the way. You can then take a taxi to Siem Reap.
By Shared Taxi: Poipet border has a lot of taxis which run to Siem Reap. There are shared taxis and are quite safe. It will not cost you also much and is a convenient option.
HOW TO GET AROUND SIEM REAP
By Tuk-tuk: Tuk-tuks are a lovely way to get around in the city. They would answer to your call from any corner and you can get anywhere in the subtle luxury of Tuk-tuks.
By Car: You can rent one and drive around or you can simply rent one to drop you off to your desired destination.
By Motorbike: This is a lovely option for calling motodrops from any corner of the City. Lots of companies rent out motorbikes for cheap on a daily basis and you can get around inhaling every inch of Siem Reap on a motor bike.
By Bicycle: Most accommodation places rent out bikes for free or very cheap prices. It is a safe way to explore the city and you will find a lot of bicyclists exploring the city along with you.
On Foot: It is a fairly small city with attractions compactly connected. You can discover the city on foot.