Objective Mode is currently a mode that only appears in the first Killing Floor. It uses it's own separate maps which are designated with a "KFO-" prefix instead of a "KF-" prefix.
All of the official Objective Maps released by Tripwire Interactive also have a corresponding regular version of the map available for Survival Mode. Custom Maps created by fans may or may not have a regular version as well. Some fans have also made Objective Mode maps out of Official Survival Mode maps, like West London.
In Objective Mode you cannot set the Game Length, but you can still set the Difficulty to any of the five difficulty levels available in Killing Floor.
During an Objective Mode map, you will be tasked with a series of tasks by an NPC, with brief breaks to visit the Trader after each one. Tasks may include things like welding an object to fix it, escorting an injured NPC, defending an area, or transporting volatile chemicals from point A to point B.
Community members are able to make their own Objective Maps using the free SDK available for Killing Floor.