Low and Slow is the technique for cooking meat for a relatively long period of time at a low temperature.
Low and slow cooking is what most people think of for BBQ. It was developed for turning cheap, tough pieces of meat into tender barbequed meat. The long cooking period allows the connective tissue in the meat to break down and turn into tasty, moist gelatin.
The temperature for low and slow cooking is usually around 225 F.
The low and slow BBQ technique is usually associated with brisket, pork butt/shoulder, and ribs.