Cambodia is a country that only recently emerged from a century of wars, famine and fear. First it suffered from post-colonial chaos (Cambodia used to be part of French Indochine), then civil war, and was dragged into the Vietnam War (although a neutral country it was massively carpet bombed by the USA), a war that helped the Khmer Rouge seize power. The Khmer Rouge under Pol Pot carried out one of the worst genocides in history, in which 2 to 3 million people (around 25% of Cambodia's population) died. The Khmer Rouge then started a war with Vietnam, which they lost and surrendered completely in 1999.
The country has some somewhat recovered since, and Cambodia is now firmly established in the tourist trail.
After being lost in the heart of Cambodia's jungle for many centuries, the massive Angkor complex contains the remains of capitals of the Khmer Empire, which controlled most of Southeast Asia at its peak. There's countless temples, after the 5th one there's a good chance of suffering "temple fatigue", so best to just focus on the top three temples namely: Angkor Wat, Ta Prohm, and Angkor Thom.
Less than an hour out of Siem Reap is Tonlé Sap, a large freshwater lake home to entire villages built on stilts. Houses are erected on rafts that can be towed to different areas depending on water levels. Visits to the village usually were done via "government" tourist boats costing $20, but I convinced a local fisherman to take me on his boat for the same price. I had to argue with a lot of people, who were really not happy that I went on a fisherman boat instead of the tourist one. But at least that way I knew the money would go straight to the fisherman as opposed to corrupt government louts. I shared the boat with the fisherman's grandmother and his two kids, they gave me a tour of the lake and took us to see their house in the village, which was a much more personal experience.
While in a Siem Reap, I took part in a Cambodian wedding, I did not really understand most of the ceremony but the outfits were very snazzy.
Siem Reap mostly exists to house tourists who come to visit Angkor Wat. There's a night market with various Apsara dance shows (traditional Cambodian dance), and there's a very lively street called Pub Street.