What gets shared between players and what is separate?

Hey guys my friend and I just started playing this game today for the first time and we're loving everything from the humor to the action. We've noticed some thing are shared between us like money and XP while our inventories are separate. We just got to Sanctuary and bought something from the black market. We assumed upgrades would apply to both of us but it seems it only affected me. Is this supposed to happen? Do we need to buy ammo and storage upgrades on our own? And what other things are shared/ separate? Thanks for the help! via /r/Borderlands2 http://ift.tt/2sC51lV