Project-13 will take you to a paralell version of the earth where there has been a nuclear war, where the lack of resources is evident, you take place as a survivor of the nuclear war, urban and countryside zones are devastated, filled with mutated creatures and a deep cloud of dirt flying through. You will have to keep your thirst, hunger and energy, stable, as well as avoiding decieses and the monsters that the nuclear war left. You can harvest (a tree for example), build stuff, interact with other players and npcs, find resources (like a empty waterbottle and fill it...)
When you go into fps mode, you get your crosshair in screen (if you have a weapon), and it goes like any other fps, you can shoot enemies in order to get rid of them.