Domestic Medium Hair isn't a breed. It's a coat description for mixed-ancestry cats whose fur falls between short and long. You'll usually see a fuller tail, tufts between the toes and a soft undercoat, but not the heavy ruff of a true longhair. They come in every color and pattern, from tabby and tuxedo to calico and solid black.
Because their ancestry is mixed, size, personality and health follow the cat rather than a breed standard. Most are medium-sized and sturdy, weighing about 6 to 14 pounds, and many live well into their teens. Expect a little more brushing than a shorthair needs, especially in spring and fall. The best guide to temperament is spending time with the cat.