the difference is how long you cook the meat. rare is nearly raw in the middle, still red and bleeding. well done is fully cooked, brown all the way through, and often dry. medium is in between, a little bit of pink and a good amount of moisture. most people won't cook a rare hamburger, because the meat has been ground up, meaning it all has been exposed to outside contaminates, but it's fine on a steak because the inside has NOT been exposed to any outside bacteria. there's also medium rare, and medium well, which are the in-betweens of these options.