PDA.

Vital for your work at the Union Aerospace Corporation.  Stores your e-mails video discs and allows you to download and access other people's PDAs which you find throughout the game.

Key card.

A key basically only fancy and electronic.

Health.

Health restores your health.

Armour.

Armour is suppose to give you an additional 50 protection.

Armour shards.

Armour is suppose to give you an additional 5 protection.

Bullets.

Gives you something to shoot with.

Clips.

Gives you something to use with your machine gun.

Ammo packs.

Gives you some ammo for your chain gun.

Cells.

Gives your plasma gun for fuel.

Rockets.

Gives you something to shoot with.

BFG cells.

Energy for your BFG.

Berserk.

Basically like in the original DOOM.  Causes the player's vision to go red and funny looking, causes the player to scream at the top of his lungs, doubles the running speed and causes 30x damage for melee attacks (fists for example).

Megasphere.

Makes you even tougher.

Invisibility.

Makes you invisible for a short period of time.

Frank's Lunch

This is the most deadly item in all of DOOM 3.  Throw his mouldy lunch onto the floor in front of an opponent and watch him pass out from the stench of it.