You can either matchmake to join someone else who requests help -- works a lot like Souls where if the host dies the session ends. OR you can essentially set up a party of 2 to take on an entire mission together, where you can sort of die and revive eachother. In both co op variants both the host and helper receive loot / experience / everything.
Thanks, sounds great!
edit: Some more questions...
- is there an option to disable the dmg numbers that pop up all the time?
- does the game somehow adapt when playing co-op? More/stronger enemies?