Campaign
These are story lines consisting of several chapters (levels, maps). At the end of each chapter, players are given awards for how well they did on certain aspects. Players must navigate through different areas that have their own Panic Events (avoidable traps, i.e. car alarms) or Crescendo Events (Unavoidable traps, i.e. alarmed doors, generators, etc.) Each campaign features a "finale," in which the Survivors call for help and must face lots of hordewhile waiting for rescue to arrive. This includes several waves of Common Infected and Special Infected as well as two or more Tanks (though, never a Witch). After holding off the Infected for a certain period of time, the remaining Survivors head to the rescue vehicle. If at least one Survivor escapes after the finale, credits will role with the Survivors' final results.
Single Player/Offline Co-OpThis is the same basic concept as Campaign, but instead of playing with other people online, you are playing alone or with a partner on the same system.
Versus
Here, players work together to complete your objectives as both the Survivors and the Infected. That means that if you're a Survivor, make it to the end of the level with as much health as possible. If you are an Infected, you need to prevent the humans from doing just that. Up to four human players can play against a maximum of four Special Infected, all of which are controlled by human players. This gameplay mode is currently the only mode where human players can choose to play as the Special Infected. Some maps are also slightly changed to make it harder for the Survivors, such as removing closets or areas where similar tactics are possible. The lobby leader chooses a specific campaign to play, then the first team plays as the Survivors while the others play as the Special Infected. The map lasts until the Survivors reach the end or they are killed. Then the teams switch roles and replay the level as the opposite team. At the end of each round teams are compared based on criteria including: if the Survivors made it to the end, what was the Survivors' health at the end, how many Survivors made it to the safe room and how difficult the map itself was. After both teams have finished the map and been scored, the process begins all over in the next level where the team that has the most total points plays as the Survivors first. Once the campaign has been played through entirely this way, the final scores for each team are displayed and the game returns to the lobby.
Survival
This mode was released in the game's first DLC pack from Valve; the Survival Pack. This gametype throws waves and waves of the Infected the Survivors until they die. There is no way to really win this through surviving, but instead the goal is to survive as long as you can and set new records. There are currently eighteen different maps to play on in Survival Mode, seventeen come almost directly from the campaign mode while a new map called The Lighthouse, which was released with the pack and is only playable in this mode, but appears to be attributed with a campaign called The Last Stand. All of the maps have been modified to essentially trap the Survivors into the designated area. Doors have been blocked off, and most choke points such as closets have been removed, leaving the players out in the open. Each survival match starts with a setup time, allowing time for you and your team to scour the map for useful items. The teams will find four health kits, tier 2 weapons, pipe bombs, molotovs, pills, propane tanks, oxygen tanks and gas cans scattered around the area. You can take as much time as you want, as the timer at the top of the screen doesn't begin until someone hits the button that alerts the horde. Once the button (typically in the form of whatever the chapter's Crescendo Event is in the campaign) is activated, the action gets off to a furious start and escalates from there.
Scavenge Mode (Left 4 Dead 2 only)
This is a new game play mode exclusively for Left 4 Dead 2. Each game consists of the Survivors and the Infected competing against each other to achieve a goal. The Survivors' goal is to find 16 gas cans hidden throughout the map and bring them back to a generator, using the gas as fuel. The Survivors will have 30 seconds to set up and grab their weapons, while the Infected attempt to find ambush spots. Once the round starts, the Survivors have 2 minutes to find the canisters, with an additional 20 seconds for each canister they find, before the round ends. The Infected team's goal, much like any Versus match, is simple: kill the Survivors as fast as you possibly can. When the round is over by any means, teams will switch sides. It is at this point that the new Survivors will have to get at least one more gas can then the previous team in order to win the round. It's recommended that the Survivors watch their aim while shooting at Infected to avoid accidentally shooting the gas can, thus destroying it and lowering the possible score. At the same time, this can be used as an advantage for the Infected team. Also, the Spitter has the ability to destroy gas cans with her acidic spit. For game play reasons, gas cans have an aura of light around them to help Survivors pin point where gas cans can be located. There are two aura colors, a white aura when they are on their original position and a yellow aura when they have been dropped by Survivors. The white aura gas cans are invulnerable to damage while in their original positions as it would be easy for a Spitter to destroy groups or easy access gas cans before Survivors have a chance to get to them. If the timer runs out but a Survivor is holding a gas can, the game goes into Sudden Death, meaning that the round doesn't end unless the Survivor drops the gas can or pours it into the generator, meaning more time will be awarded.
Realism Mode (Left 4 Dead 2 only)
This mode removes the auras usually shown around players through walls. Also, the auras around items are only visible within a few feet, making the game require more communication and teamwork. Realism mode also makes body shots less effective at dispatching the Infected, while making head shots the only efficient way to kill infected. It can be played on any difficulty. Teammates die instead of being downed first (this includes with the Witch) and rescue closets are also removed, making the defibrillator the only way to revive teammates. Between the dependence on headshots and the number of Witches around, melee weapons are more useful that usual.
Realism Versus (Left 4 Dead 2 only)
This game mode made it's debut as the first Mutation. It was so well-recieved, that VALVe then had the players vote, via the in-game blog, on whether or not to make it a permanent game mode. The vote passed and a few weeks later, it was added to Left 4 Dead 2's main menu. This game mode is exactly as it sounds. Play with 7 other players in a Versus match with Realism rules in full effect for the Survivors. That's right, they can't see each other's glows but the Infected team still has that ability. The Survivors try to stay as close as they can while the Infected team tries to split them up in order to do the most damage. This mode is for the hardcore team players who need the extra challenge.
Mutations (Left 4 Dead 2 only)
Along with The Passing DLC for Left 4 Dead 2 came a new game mode called Mutations. The concept is that there is a new Mutation (game mode) each week. Some are playable in campaign mode and some are playable in versus mode. This adds a consistantly new and fun experience for the gamers. After four mutations, we get to vote for one of the last 4 to return the next week.