There are currently (as of 1.2.02) three game modes to choose from. No Survivors(Easy), Road Kill(Medium) and Deadline(Hard).
This game mode starts off slower and progresses slower than other game modes. There is not time limite for no survivors and you can recruit new survivors into your group, but you will only start with one. In addition, there are no aggressor dilemmas and some buildings produce resources such as ammo, medicine or food.The no survivors game mode was added in the 1.2.0 update.
This game mode has the objective that you must find a car, the parts for the car and then repair the car before driving off. There is no time limit and you will start with five survivors, but you cannot recruit more survivors into your group. There are also no buildings in which produce food.
This game mode has the objective that you must find information necessary in getting rescued and then arrive at the location in question after retrieving certain items. You start with five survivors and cannot recruit new survivors and you have a time limit of 5 to 7 days before the rescue helicopter will arrive. This game mode has the most aggressor dilemmas and is the hardest of the three.