You start with nothing, in a generated, infinite world. You attack little blocks with your hands. Hey, you got a bunch of wood, maybe you can make a crafting table? Now you can craft an axe or pickaxe or shovel or sword by putting these rocks in a certain way. Cool, nwo you can mine better stuff. Oh fuck, zombie. Better craft armor. You lost some health. Better go kill a pig and get some pork. Pork works better cooked, so you build a furnace, put some wood or coal in it... mmm cooked pork.
Its like that, but with way more. I don't play that much anymore, but when I got it, I easily played 45 hours in my first week.