There are four modes in the original Left 4 Dead-campaign Single Player, Campaign multiplayer, Survival, and Versus.
In Campaign modes you play as one of the four survivors. There’s an offline single player version as well as an online co-op version, where the difference is how many survivors are bot-controlled. There are five “campaigns” or main story sections in L4D (with one being a free PC add-on distributed on Steam after release), each with usually four to five “chapters”. Each chapter consists of a starting “safe room” where the group can re-arm, heal, and resupply without zombie interruption. The goal of each chapter is to reach the opposite safe room at the end of a directed path-usually a path made from everyday streets.
The first campaign starts off with the group barely making it to a rooftop after getting overwhelmed by zombies. From there they fight their way through chapters filled with streets, a subway system (don’t worry, it’s just one sub-chapter), and eventually to the rooftop of the closest hospital-a hospital that happens to have a lot of floors. The last chapter of a campaign has the campaign’s “finale”-a recurring showdown mechanic where the survivors have to fend off waves of zombies and survive long enough to escape to the next campaign (story-wise). Finales exist like this at the end of all campaigns. Side note-“mini-finales” are usually spread out through each chapter as well-smaller choke-pointed obstructions the survivors have to pass against the will of waves of Horde and Special Infected. These are normally designed in a way that seamlessly integrates into the story-shocker!
A new twist to Campaign mode was added in Left 4 Dead 2: Realism mode. Unlike normal gameplay modes where weapons, health items, and even teammates are highlighted to make them easier to see, Realism mode removes all of these. This forces teammates to stick together more and forces more energy into exploration (for items that assist in survival).
Survival mode is similar to campaign mode but focuses solely on the mini-finales, usually ones located near a safe room. The only difference between Survival mode and Campaign mode is that Survival mode is timed, the map is confined to the mini-finale area, the enemies come in very large number, there’s a survival timer, and the goal is to survive as long as possible.
Versus mode is like campaign mode also, where teams take turns moving the survivors from safe room to safe room. While one team does this, the other team tries to prevent them through Special Infected play. Scores are centered around how far each team makes it as survivors to the safehouse, with bonuses for actually making it into the safehouse and a tiebreaker for each round dictated by damage done as Special Infected. The nuances of survivor/SI abilities and interactions really pronounce themselves in versus. For example, chargers/jockeys have the potential for being the most lethal characters on the map depending on layout. Spitters do well when they can be guaranteed survivors will stay in their spit for any amount of time (other SI attacking, choke points, etc.) Sounds play such a role in versus that it can either be a detriment to SI by giving away location or can be a weapon (lingering behind the group with an SI plays like psychological warfare). L4D versus mode might come across flimsy at first, but after giving it a fighting chance it comes across like “FPS chess”. After this long playing it, I’ve really got to hand it to the dev staff for their mostly great SI ability design.
Versus mode also allows for deep customization through modding. While the standard maps in L4D/2 pose balance issues at times, mods allow for more than just the normal four-vs-four using default game rules. Viable six-vs-six, eight-vs-eight, and ten-vs-ten mods exist, and these often bring extras to help with game balance like points systems, item/status purchase abilities, persistent stat-tracking, the list goes on. These modes have proven so deep that they really deserve their own classification as a separate game mode.
Two new game modes were added in Left 4 Dead 2-Scavenge Mode and the newer Mutation mode. Scavenge mode is a versus mode that focuses on the collecting challenges that make up many of the mini-finales in L4D2. For example, in the first campaign “Dead Center” (a tribute to Dawn of the Dead!), the finale consists of the survivors collecting gas cans from around different levels of the main mall area and filling up Jimmy Gibbs’ race car with them. Once the team collects the required amount and meets back at the car, they pass the finale by getting into the car and racing forcibly through the front doors and over corpses in the way. Scavenge-like challenges sound like mechanical cop-outs, but in a world like L4D it’s a great mechanic-it gets the survivors mobile which creates dynamic confrontation environments that suit SI design, and collectables placement along with level design make retrieving things more than just collecting gold keys. With scavenge challenges spread throughout the campaigns and working so well, Valve decided to make a versus version of it where teams swap off as survivors and SI and try to better scores under a time limit.
As one of the post-release enhancements to the game, Mutation mode provides a regularly-changing (monthly-ish?) mode that so far has been a different twist on one of the existing gameplay modes. For example, one of the first mutations was realism versus which combined normal versus with realism mode for the survivors. Some people argue that survivors are overpowered compared to Special Infected in versus modes (to the point of promoting mods aimed at balance), and this mode arguably addresses that to an extent. Some modded versus servers run Realism Versus continuously now, by the way.