As in many Buddhist countries, the traditional Cambodian greeting is palms together as in prayer with hands in front of the chest and head inclined slightly in a bow. The degree of respect is indicated by the height of the hands and the deepness of the bow. In Khmer this is called Som Pas.
As foreigners, most Khmer will not expect us to necessarily be familiar with their system of etiquette but will be impressed if it is demonstrated by us. Moreover, familiarity with the general principles can help avoid causing unintended offence.
When meeting, especially with social superiors, Cambodians traditionally Som Pas while saying Choum Reap Sur (Hello). When leaving, they will again Som Pas while saying Choum Reap Lir (Goodbye), usually as a display of respect. If you are greeted in this way, it would be impolite not to return a Som Pas as it would be tantamount to us rejecting an offered handshake in our culture.