Over the border in Vietnam, mam is used as a catch-all fermented aquatic animal word: nuoc mam is fish sauce; mam tom is shrimp paste; bun mam is purported to be the best noodle soup in Saigon.
On my Cambodian side of the border, mam is mam. It refers to the above salted, fermented fillets of snakehead fish, to which roasted red sticky rice and palm sugar are added during the fermenting process to impart an earthier and sweeter flavour. The sugar and rice also lends the ingredient a reddish tinge. From the time that the fish is filleted, mam can take over a year to reach maturity. According to the unsubstantiated rumours that I transcribe as actual history, mam originates from Kampuchea Krom territory, the wedge of the Vietnamese Mekong Delta that was previously under Cambodian ownership.
What to do with it? You ask with veiled incredulity. Being the crème de la crème of rotting yet edible aquatic vertebrates, mam is versatile. Like the more pedestrian prahok, it’s added to soups, noodles, or steamed on its own but unlike it’s poorer grey fermented brother, mam adds far less pungency to dishes and a little more fishy subtlety.
Addendum (2 November 2006): Changed “mam nuoc” to the correct nuoc mam. Sorry.