Disaster Recovery (DR) is the process of restoring systems to an operational status after a catastrophic systems failure leading to a complete loss of operational capability.

Business Continuity (BC) is the forethought to prevent loss of operational capability even though there may be a catastrophic failure in some parts of the system. BC includes DR plans to restore failed system components.

There will always be failures, but, with the proper systems design and operational planning, there can be significantly reduced risks of operational degradation. 

Simply put, design the system with the idea that one or more components in the system may fail and each of those failed components should have a backup to take over.  The topic can be very complex or relatively easy, but hardened systems even demand a geographically separate backup location, usually in another state.