Slab Doors are what we normally think of when we think of doors: just a rectangular slab of wood, composite, or steel. It has no hinges or hardware. It's up to the owner to attach it to the door frame via hinges.
Some factors that may influence your decision to install a slab door:
Slab door installation requires a steady hand, a good eye, and lots of practice. You'll probably be fine if you're doing a one-for-one installation. That is, if the door you are installing is exactly the same size and has the same configurations (screw holes for the hinges, door knob sizing and positioning, etc.) as the door you are removing. Any variation in door size and configurations, and it will be difficult, if not impossible, to make it hang right.