As of July, 2022, Cambodia has dropped Covid-19 vaccination and quarantine requirements for travelers (with one caveat). There are still quarantine requirements for unvaccinated travelers who test positive on arrival, although they have been relaxed from previous quarantine requirements. Most land borders have now been re-opened.
Those who are fully vaccinated (two doses of a vaccine, or one dose of a single-dose vaccine) do not need to take a Covid-19 test on arrival. You will need to present proof of Covid vaccination, in the form of a card of certificate on arrival.
Instead of a mandatory quarantine on arrival, unvaccinated travelers and those who have had only one dose of vaccine will need to take a $5 rapid test on arrival.
Those who are positive and showing mild symptoms have the opportunity to self-quarantine at a private residence (this will probably only be possible for expats and residents), and others who test positive can quarantine at a government-approved hotel at their own expense.
Travelers who test positive and show more serious symptoms will need to quarantine at an approved hospital at their own expense (and this can get expensive, with a minimum cost of $235/day, so consider getting insurance before you arrive — get a free no-commitment quote).
What does this mean for tourists? It would be advisable to take a few Covid tests before boarding the plane, and wear a mask on the plane, as you don’t want to risk testing positive on arrival and having to pay for an expensive quarantine hotel.
For more information, see the very (overly?) long and detailed Cambodia visas page.