French for "under vacuum," sous vide is a cooking process in which food is encased in an airtight plastic pouch (typically vacuum sealed) and cooked for a long period of time at a (precise) low temperature.
Using traditional methods of cooking, you might put a steak on a 750 degree grill, attempting to get the center of the steak to a perfect medium-rare temperature of 130 degrees, without cooking the outside of the steak until it's gray and lifeless. To make it even more difficult, even when you take the steak off the grill, the temperature of the center continues to increase due to the heat of the meat surrounding it.
The magic of sous vide is that you cook the entire piece of meat at the precise temperature you like. To cook a steak to the perfect medium-rare temperature of 130 degrees, you cook the steak in 130 degree water. It takes a lot longer to get a steak to 130 degrees by cooking at 130 degrees, but the benefits are worth it.
1) It's impossible to over-cook. No part of the steak can get over cooked.
2) The entire steak, from "coast to coast" is exactly how you like it.
3) Timing is easy. I usually cook my steaks for somewhere around six hours. If your guests are late, an extra hour (or three) doesn't make any difference.
4) The fat in the steak is always perfectly rendered. It's absolutely amazing how great inexpensive cuts of meat turn out when cooked sous vide for six hours.
There are many sous vide cookers out there. I'm more of a do-it-yourself (cheap) kind of guy, so I built my own sous vide cooker for less than $40. The fancy, $500 cookers have water circulators and tenth-of-a-degree precision, but from my experience, that isn't necessary. For $40, you can make absolutely incredible steak!