In Left 4 Dead 2 you shoot fast moving infected style zombies, with an additional 8 that have special attacks. The game is divided into 8 campaigns spanning 3 to 5 maps each. You do not need to have played the first as the story is the same in both games. The characters and environments are different because the games take place in different areas of the country.
Environments include, a hotel, a mall, a carnival, a swamp with town, a town and sugar mill during a heavy rainstorm, an area around a vertical lift bridge (where you briefly meet the Left 4 Dead 1 characters), a city and hospital (campaign taken from Left 4 Dead 1), and an area leading to the previously mentioned vertical life bridge. The last 2 you actually play as the Left 4 Dead 1 characters.
You play one of 4 different survivors trying to get to a safe area. There are also team based versus style game modes that will allow you to play as one of 7 of the special type of infected against the 4 survivors, but you can't choose which you play as, and it changes every time you respawn. Versus modes swap you from survivors to infected at the end of a round. So you cannot play an entire game as just one or the other.
Scavenge is a versus style mode where the survivors collect gas cans with a limited amount of time. Survival is just the survivors trapped in an area to see how long they can survive.
Games can be well over an hour in length. Depending on game mode, difficulty, and campaign.