There are 5 Game Modes for Left 4 Dead 2:
-Campaign(Online/Single Player)
-Versus(Online)
-Scavenge(Online)
-Survival(Online)
-Realism(Online/Single Player. This mode applies to all the 4 modes above)
-Campaign Realism -Versus Realism -Scavenge Realsim -Survival Realism
Campaign allows you to choose 5 different levels to play either online or single player.
Versus allows you to play as infected or humans online (4 v. 4), playing on a chosen level.
Scavenge allows you to play as infected or humans online (4 v. 4), playing on a chosen level. In this mode, humans try to gather up supplies while being hunted by the infected.
Survival allows you to play as infected or humans online (4 v. 4), playing on a chosen level. In this mode, humans try to survive as long as possible while being hunted by the infected.
Realism is a harder version of all of the above modes, which means there will be no indication where your team mates are.