Its essentially the prequel to Guns Online. It follows the journey of Gabriel, aboard his ship Icarus, and you must defend it, and its cargo, using various guns in which can be upgraded for any cargo that you managed to defend. Hence the name Guns of Icarus.
Now, within the Guns Online story, it is completely unknown as to why Gabriel takes the journey that you will find he takes if you play the original
I highly recommend it. Quite addictive. It is a kind of adventure mode, as you travel to towns and as you do, get better guns (assuming you survive)
Also, buying this game yeilds the "Fallen Hero" Cap which was worn by Gabriel (i think)
It has coop multiplayer yes. Basically instead of just Gabriel running around shooting and repairing, you can have some others join you aswell, making the journey easier of sorts
Good Buy. Great Game. Keep in mind its not nearly as fancy, but it gives you an insight into the towns, and the man himself.
Buy the Game, Fly the Name. Guns of Icarus
it does have some drawbacks, being that it was before all the fancy graphics and stuff. There is no way to steer (which makes sense though) and you are defending the whole ship by yourself. But all in all, if you are a fan of Guns Online, this will give you the insight you might need into the world of GoI