Tortoiseshell isn't a breed but a coat pattern: black-based and orange-based colors mixed in patches or a fine mottle, usually with little white. Dilute torties show soft gray and cream, and torbies add tabby stripes. Nearly all are female, because the orange gene sits on the X chromosome.
Most torties you'll meet are domestic shorthairs, though the pattern also appears in breeds such as Maine Coons and British Shorthairs, so size, coat length and energy vary. Plan on a medium cat of roughly 6 to 14 pounds, moderate shedding and a lifespan of 12 to 18 years. The famous "tortitude" is mostly folklore: a cat's age, history and socialization say far more about personality than its colors.