Main Ammo: The number of shots you've got left in the weapon you are currently using.
Health: Percentage of health remaining (0 to 100).
Arms: If the number above is highlighted, the corresponding weapon is available and you can choose that weapon by pressing the number on the keyboard.
Fist / Chainsaw.
Shotguns / Combat Shotguns. Pressing the "3" key will toggle between the Shotgun and the Combat Shotgun.
Chainguns.
Rocket Launchers.
Plasma Rifles.
BFG 9000s.
This is your mug. When you're hit, your face looks in the direction from which the damage came. Also, as you take damage, your face begins to look like raw hamburger.
Armor: Your armor helps to protect you as long as it lasts.
Key Cards: You can see any keys you possess right here.
Ammo: This shows how much of each type of ammo you're carrying and the maximum amount you could carry.