Cambodia is best known for Angkor Watt, which is indeed incredible (see photos from Siem Reap below). There's a huge amount of poverty, even for this part of the world, which can make things hard going at times. Be prepared to deal with many beggars in the bigger cities (Siem Reap and Phnom Penh).
While I found Cambodia a little hard going, others have assured me that it's just this way in the bigger tourist destinations; away from these areas, it's a different world. I plan on returning to find out for myself.
Visas on arrival are available for most nationalities at most borders. An exception to this is the border between Cambodia and Laos, which is always opening and closing; if you wish to cross here, please acquire a visa before hand.