Most methods of cooking have been around for hundreds (or thousands) of years, dating from the first time a caveman dropped a mammoth steak onto a fire and liked the result. New approaches to cooking don't come along often, but sous vide cooking is one of the exceptions.
First used in the 1970s, sous vide cooking is about using a better way to heat food. Rather than apply heat directly (such as frying) or using a medium such as air (in roasting), the food is packed in plastic and immersed in water. The air is removed from the bag (hence the name, French for "under vacuum") so the heat can go straight from the water into the food. Using circulating water rather than air means that the temperature can be very precisely controlled: a good sous vide system can maintain the temperature of a pot of water to within 1 degree Fahrenheit over several hours of cooking time. Some manufacturers claim better than this: Anova Culinary says that its devices have a temperature stability of plus or minus 0.01 degree.
The pro of sous vide: Control
This ability to very carefully control the temperature is the secret of sous vide cooking. Ovens produce a huge range of temperatures as the heat is turned on and off. A sous vide system brings the food to the desired temperature, and keeps it there. This means you can be much more precise about how the food cooks, which makes a huge difference to the cooking process.
It makes a difference because small differences in temperature make a large difference in cooking. In eggs, the proteins are broken down (denatured is the word that scientists use) at temperatures between 130 and 160 degrees Fahrenheit (about 54 to 71 degrees Celsius), with the process accelerating as the temperature increases. If you cook an egg sous vide style for 30 to 40 minutes at the low end of this scale, you get a runny, clear white (cooks refer to this as a loose white). Increase the temperature to 140 degrees F or 60 C, and the egg white starts to solidify and whiten, while the yoke remains soft and runny. Bump this up to the higher end of the scale (above 150 F, 66 C) and the yoke starts to solidify as well, giving you the more conventional hard-boiled egg.
For vegetables, sous vide preserves more of the taste of the veggies because they aren't being blanched by hot water; the precise temperature control means that you can cook, but not overcook them. Plus, the nutrients and other tasty chemicals in them aren't being leached out by the water: because the food is sealed in a plastic bag, the nutrients stay in the veggies. Again, having a range of precise temperatures available gives you control over how well done the veggies are.
Sous vide cooking also works well with meat, because the texture of the meat can be preserved. When used properly, the cooking process can break down the collagens that make it chewy and unpleasant, but preserve the muscle and other parts that provide the taste. That means you can get the lighter texture of cooked meat, but still preserve that raw taste of rare steak. The vacuum packing phase also means that sauces and other flavors will move into the meat more effectively, and the juices that come off the steak while cooking can be kept and used in a nice sauce.
The con of sous vide: Speed and safety
Sous vide does have its downsides, though, it is slow, and requires some caution to use safely.
Because sous vide uses lower temperatures than other cooking methods, it takes longer. Cooking an egg can take 40 minutes or longer, and a nice thick steak can take many hours of cooking. So, it isn't something where you can whip up a quick lunch or snack: you have to prepare the food long in advance and live with a long cooking time.
Safety can also be an issue, especially with food like meat and eggs. In conventional cooking, the U.S. Food and Drug Administration recommends that foods like chicken are cooked to an internal temperature of about 160 degrees F to kill bacteria and other nastiness. Sous vide cooking usually doesn't reach that temperature, so you need to cook for longer to make sure that the bacteria has been dealt with. While 160 degrees F will instantly kill most salmonella in chicken, the USDA says that a sous vide setup running at 140 degrees would need at least 25 minutes to wipe the salmonella out. Again, this is usually a question of making sure that you cook for long enough, so make sure you follow the manufacturer guidelines.
Sous vide cooking is a great example of Appliance Science, of how a technique enabled by science and designed for commercial use made its way into the home. And, perhaps more importantly, the results taste really, really good.
Correction, Thursday 4:30 p.m. ET: Edited to clarify the safety guidelines in the paragraph beginning, "Safety can also be an issue". Our thanks to the commenters who pointed this out and offered advice.