Armour class
When something injures you, your Armour class (often abbreviated to AC) reduces the amount of damage you suffer. The most common way to improve your AC is by wearing body armour, gloves, boots, cloaks, and head gear. Other pieces of gear, especially artefacts, may also increase your AC, and magical spells may temporarily boost it too. While heavier armour provides superior protection, it also reduces your Evasion, hinders spellcasting, and causes you to occasionally miss attacks unless you have sufficient Armour skill to offset its penalties.
Armour can be enchanted with scrolls of enchant armour to grant bonus AC.
AC damage reduction
AC reduces the damage of attacks susceptible to AC by an absolute value between 0 and your current AC value: in other words, an AC of 10 will reduce damage by a random number between 0 and 10. However, this is not strictly true, as there is always a minimum guaranteed damage reduction based on your body armour's base AC. See the AC calculations page for more details.