In best-effort delivery world, IP packets may be dropped, arbitrarily delayed, reordered, corrupted, or delivered, if the network feels like delivering them.
The only expectation that application and protocol writers may have about network's behavior is that some packets once in a while will get through. (If not, connectivity is lost.) All errors are corrected end-to-end.
Amazingly, there are protocols that can make very good use of networks like that. TCP is an example.
Best-effort philosophy contrasts sharply with how the phone system works, but allows to build significantly cheaper networks. Now we have whole generation of engineers for whom best-effort is the natural way a network functions. IP QoS is a compromise endeavoring to attain the best of both worlds.