French for under vacuum. The process basically consists of vacuum packing food in a plastic bag and immersing it in water bath at a constant temperature (typically the temperature of the food when it's considered "done") for a long enough period of time to let it reach that heat all the way through and kill any possible pathogens in the food.
Most food, when cooked sous-vide requires "finishing" or a quick trip into the frying pan or under the broiler to give it the appearance of having been cooked by that method.
Two of the biggest benefits of sous-vide are that the food is extremely moist since all of the moisture is trapped in the bag with the food and the food can rest at the "done" temperature for many hours without becoming overdone, allowing for wide flexibility in serving times.
There will be much more to come on this topic.