Pastrami is made with beef brisket. Beef brisket comes from the lower part of a cow's chest. The brisket is below the chuck and above the shank.
Pastrami is made by smoking corned beef. The process of smoking adds flavor to the meat. Corned beef is made by soaking beef brisket in a salty brine, along with sugar and spices, for several weeks. Before the invention of refrigeration, soaking a meat in brine was one of the two methods for preventing meat from spoiling. Adding salt to the meat kills all the bacteria and preserves it for a long period of time.