Angkor Wat itself means city of temples. It's the best known temple of Angkor. It is also printed on the Cambodian money bills.
Angkor Thom is known for its buddha faces which are engraved in the pillars of the temple so that you have one buddha facing in each direction of the sky. The temple was built by Hindus. That's why there can be found lots of Shiva goddesses all over the temple. The best way to start with the temple is through the south gate. Most of the buddha heads are still intact here.Angkor Wat is still used for worshipping amongst buddhist monks. Sometimes you can watch them taking photos like other tourists, too.
Ta Prohm lies deep down in the jungle. 
That's how all temples looked like before they started the excavation process we were told. This is the place where 'Tomb Raider' was filmed. The locals call it 'Tomb Raider temple' after the film.
If you are interested in details. There is a direct flight from Hong Kong to Siem Reap with Siem Reap Airways. It takes 2 h 20 min and the flights are Mondays, Wednesdays and Fridays. The return flight has a stopover in Phnom Pen. That takes a bit longer (including waiting time 5 h 30 min). From November to February/March it's high season because the temperatures are not as high as during the rest of the year.
That's how all temples looked like before they started the excavation process we were told. This is the place where 'Tomb Raider' was filmed. The locals call it 'Tomb Raider temple' after the film.If you are interested in details. There is a direct flight from Hong Kong to Siem Reap with Siem Reap Airways. It takes 2 h 20 min and the flights are Mondays, Wednesdays and Fridays. The return flight has a stopover in Phnom Pen. That takes a bit longer (including waiting time 5 h 30 min). From November to February/March it's high season because the temperatures are not as high as during the rest of the year.
But if you don't have problems with hot weather you should go there in the low season. It's a lot less crowded and more relaxed then. If it rains it rains usually in the afternoon. Usually the rain is quite heavy but eases relatively quickly. When we went there in June it didn't rain at all. There are lots of hotels. The best is to take one in Siem Reap town. We had a hotel which was a little bit farther away and had to take a tuk tuk to get into town. It was about 4 US$ each direction. But the driver will wait for you the whole evening. That's included in the price.
Siem Reap has a small centre with an old market and a so called bar street. You can have a couple of drinks there for just a few US$. A beer was 0,75 US$ for example.I had imagined that Cambodia was a poor and little developed country. In one newspaper I read that 35% of the people have less than 50 cents per day for a living. But it really didn't look like that at all. Well, it's poor for sure but the people seem to be happy and they are very friendly and lot's of them speak really good English. They have a public school system and for the booming tourist market they offer each and every convenience you can find anywhere else. It's probably completely different in the less touristic country side.
Of course you will find slums there as well without sanitary installations but there are also many chic restaurants, a bar scene and many many new cars. There seems to be a middle class which makes good money with the tourists. Unfortunately the country is so corrupt that there is not much left over for the poor. E.g.the tickets for the temples are sold through a Vietnamese company. They only have to pay a small maintainance fee to the Cambodian government. This is a relict of the war when Vietnam helped Cambodia to destroy the Khmer Rouge. But most of the Cambodians are unhappy with that deal.