Survival Mode is the traditional mode in almost all Killing Floor games (though it was called "Killing Floor Mode" in the first Killing Floor).
It involves the players going solo or with a team against waves of specimen and ultimately against a boss. You are usually able to choose between Short (4 waves + Boss), Medium (7 waves + Boss) or Long (10 Waves + Boss) game lengths, as well as choosing the Difficulty. In the first Killing Floor, certain achievements can only be earned by completing long matches.
In Killing Floor 2, some waves will have a bonus objective called "Stand Your Ground." When this occurs, you will be tasked with keeping specimen out of a designated area. The more the specimen invade the area, the less reward you'll get at the end of the wave. Rewards include Dosh, Experience, and Vault Dosh.
Holdout-Style Survival Maps
Killing Floor 2 also added a new twist with Holdout-Style maps (not to be confused with the Holdout Mode in Killing Floor: Incursion). They are played in Survival Mode like other maps, but have you moving to entirely new areas between waves until you finally reach the boss arena.
They can be played in any Game Length setting, but you may not see all available areas for a given map unless you do a Long run.
Other than having to move to new areas, they otherwise play like regular Survival maps.
The Descent was the first Holdout-Style map added to the game.
Objective-Style Survival Maps
Killing Floor 2 also added Objective-Style maps (not to be confused with Objective Mode in the first Killing Floor). Like Holdout-Style maps, they are also played in Survival Mode just like any other map. The difference is that there will be optional map-specific objectives during certain waves. Similar to the "Stand Your Ground" event that can spawn on regular maps, but can include new twists like having to use your Welder to repair things.
Airship was the first Objective-Style map added to the game. It features four optional objectives that always appear in the same order. If you play a short game, you will have an optional objective every wave.