In DOOM, there were dozens of pick-ups you could find, and they very often were detrimental to your survival. And here they are.
AMMO: These items are for your weapons.
AMMO CLIP- Gives you 10 bullets. Found lying around, or could be obtained by killing a Former Human. Bullets can also be obtained from Boxes or picking up a Chaingun.
BOX O' BULLETS- Gives you a crapload of bullets. About 100, I think. Less frequent than clips.
SHELLS-Gives you four Shotgun shells. Shells can also be obtained by picking up a Shotgun of a Box of Shells.
BOX OF SHOTGUN SHELLS-Gives you 20 shells.