See also Tank Gear.
Armor Class is very important for fighting. This number indicates how good your overall armor is. This includes spells, all equipment worn, dexterity, and whether you're standing, resting, or sleeping.
The AC value shown by the identify spell is a positive rating for a piece of armor. The HIGHER this number is, the better. Modifiers on armor generally say something like: "modifies armor class by -20" or something similar. The LOWER this number, the better. That is, -30 to AC is better than -20 to AC.
When you reach a high enough level (25), you will see an AC rating on your score screen. This rating takes into account all spells, equipment worn, dexterity, etc to form a total AC. The LOWER this number, the better (ie, -300 is better than -50).
The levels of armor are as follows:
WORSE than Naked Armored Angelically Armored Defenseless Well Armored Divinely Armored Almost Defenseless Greatly Armored Invincibly Armored Barely Armored Strongly Armored Indestructibly Armored Slightly Armored Heavily Armored Impossibly Armored Somewhat Armored Superbly Armored Supremely Armored Sadly Armored Terrifically Armored Sniktly Armored Almost Armored Heroically Armored Ironhand Armored