MINIMUM SPECS/ PC
Disk Drive : DVD-ROM
DirectX : 9.0, Mac OS X will use OpenGL 3
Hard Drive Space : At least 7.5 GB of free space
Operating System : Windows 7, Vista, Vista 64, XP, MacOS X 10.6.4 or higher ( Snow Leopard Graphics Update required )
Processor : 3.0 GHz P4, Intel core 2 duo 2.4GHz or better recommended
RAM : 1 GB for XP / 2GB for Vista, Mac
Video Card : nVidia GeForce 6600 or higher / ATI Radeon X800 or higher

Left 4 Dead 2 includes 4 game modes:

1. Campaign: up to four human players fight against the Infected to make their way through Campaign stages; any Survivor not controlled by a human player is controlled by the computer. A Singleplayer mode allows players to play without intrusion from human players.
2. Versus: up to four other human players take control of various Special Infected to try and prevent the Survivors from completing a stage. Special Infected are randomly assigned to Infected players; they cannot control the Witch or any Common Infected. Occasionally, as determined by the AI Director, certain players will become the Tank. The two teams swap sides once per stage, and are scored based on their stage progress as Survivors. If both teams make it to the saferoom with all four Survivors, a 25-point tiebreaker is awarded to the team that dealt the most combined damage as Special Infected.
3. Survival: a timed challenge where the Survivors are trapped in a section of the campaign maps, and try to survive as long as possible against an unending onslaught of Infected.
4. Scavenge: a new 4-on-4 mode that requires the Survivor players to collect and use as many fuel cans scattered about a level to fill up a power generator, while the Infected players attempt to stop them.

Left 4 Dead 2 also features a Realism mode, which can be enabled at any difficulty for either Campaign or Versus. This mode removes some of the video-game aspects from the gameplay: Survivors cannot see each other's silhouettes, and dead teammates can only be revived with defibrillators and will not respawn later in the level.Weapons and other items will only glow when the player is within a few feet, forcing the players to search the levels more thoroughly. Damage dealt to Infected is altered, with headshots dealing more damage to enemies, whereas limb or body shots require more shots, these changes can make gameplay more challenging.In addition, the Witch enemy can kill any Survivor she would have normally incapacitated. Designed to force players to work closely together and rely on voice communication, Valve created Realism mode to give players a way "to be challenged as a team" without having to increase the difficulty level of the game.

1. No Mercy
2. Crash Course
3. Death Toll
4. Dead Air
5. Blood Harvest
6. The Sacrifice
7. The Passing
8. Dark Carnival
9. Dead Center

http://en.wikipedia.org ,[URL=left4dead.wikia.com/wiki/Left_4_Dead_2 ]left4dead.wikia.com/wiki/Left_4_Dead_2 [/URL]