Some of Angkor's sites were originally built as Hindu temples, while some were built as Buddhist temples, and yet others were converted over the years. Today, most of Angkor's major temples house at least a few Buddha statues (nearly all added later) and draw a steady stream of monks and worshippers. You may be approached for donations, but you are under no obligation to pay unless you actually choose to accept incense sticks or other offerings.
Because these are still holy spaces for the Khmers (Cambodian people), it is best to follow the dress code of "long pants/skirt and covered shoulders." This is the dress code that the Khmers follow when visiting any temple or holy space.
By local regulation, motorcycle and tuk-tuk drivers must at all time wear a numbered vest when on the job, which goes a long way towards preventing hassles and scams. However, it's unadvised for women to travel alone, especially after dark and in the more secluded temples.
Whilst visiting the temples, beware of off-duty police officers, who are in uniform, that start walking beside you and start showing you around the temples. At this point either say that you would like to see the temples yourself, or agree on a price at the start. Several people have been requested for a fee of over $10 at the end of the temple tour and you are not going to argue with a member of the police force! The official wage for a police officer is very low, so they can easily double their salary by being tourist guides.
Whilst at the temple beware of anyone offering you incense. They will hand you the incense and then "teach" you a blessing. They will then ask for a donation (generally about $10) for the monks and the upkeep of the temple. None of the funds will make it to either of these causes, so it's best just to say a quick "No thank you" when they try to give you the incense in the first place.
Be prepared for vast numbers of peddlers who linger around temples. It may feel difficult or rude to ignore the constant come-ons to buy souvenirs, photocopied guidebooks, t-shirts, and assorted junk, but it can be necessary in order to enjoy your visit in semi-peace.
Touring the temples is a hot and sweaty job, so bring sunblock and keep yourself well hydrated. Some of the temples, notably the uppermost level of Angkor Wat, require climbs up very steep staircases and are best avoided if you suffer from vertigo or are not fully confident of being able to keep your footing.
Malaria is not endemic around the temple complex; however, it is recommended to seek medical advice before you travel as conditions may change.
Don't feed or approach the monkeys who lurk around some sites: many are ill-tempered and will bite at the slightest provocation.