Answer:
A cracked rib is only partially broken; the rib is still mostly intact and able to do its job, and doesn't need (or have) any special treatment except not doing what cracked it in the first place. A broken rib is more dangerous, as there are two loose ends floating around that may pierce internal organs and do a lot more damage to you. It requires medical attention to ensure this does not happen, but there is not much more that can be done than keep an eye on it.