The Word Guide
Here's a list of terms related to BG2. It's not necessary to know them but it will help you understand the way it works and calculate hits, misses etc.
THAC0 Armor Class (AC) Ability Scores/Prime Requisite Level Drain Strongholds Magic Resistance (MR)
A round is six seconds actual time and a turn is ten rounds. These terms will be used to talk about aspects such as how long it takes to cast a spell or how many attacks a characters has per round. You don't need to know these but it's very helpful if you want to get into the game and understand it better.
This acronym stands for "To Hit Armor Class 0" which is basically how much your character needs to roll to get a successful attack. A lower THAC0 is better than a higher THAC0. Basically the computer rolls a 20 sided die in the background and if your character's THAC0 is better than this number then he will get a successful hit on someone whose AC is 0. If the person has a different AC then subtract or add accordingly. For example if your character has a THAC0 of 10 and you're facing a monster with an AC of -4 then you will need to roll a 14 or better to hit that monster. If you have a THAC0 of -1 and you're facing a monster with an AC of 5 then you will need to roll a -6 or better to hit the monster. You'll always hit this monster unless you roll a 1 which is always a miss (Critical miss). A roll of 20 is always a hit (Critical hit).
Armor Class (AC)
This means the amount of defense your character has. It is rated from 10 (The worst) to -10 (The best) (Please note: AC can get below -10 with special armor and items). There are many ways to lower it such as wearing armor, shields, certain bracers as well as other magical items such as Rings of Protection. Having many points in dexterity also improves your Armor Class. There will be a small shield icon in your inventory screen which displays your current Armor Class.
Ability Scores/Prime Requisite
These are the numbers which determine strength, constitution, dexterity, intelligence, wisdom and charisma. You will get to modify these while creating a new characters. In-game characters already hav pre-made ability scores which one cannot change (Unless you used a modifying program and cheated of course). Depending on the class you choose, you should max out your character's prime requisite:
Fighter : Strength
Ranger : Constitution, Strength
Paladin : Charisma, Strength
Cleric : Wisdom
Druid : Charisma, Wisdom
Thief : Dexterity
Bard : Charisma
Mage : Intelligence
Sorcerer : Intelligence (Technically there is no Sorcerer prime requisite)
Barbarian : Constitution, Strength
Monk : Strength (Technically there is no Monk prime requisite)
Several creatures can level drain your character. To avoid this, use the priest spell Negative Plane Protection. To negate the effects of this, one can use a scroll of Restoration, the Cleric spell Restoration or try the services of a temple.
Depending on your class, your character will get a chance to manage a stronghold. The stronghold gives you a chance to get more experience points, make money or even make custom items. For all strongholds, a quest must be completed to obtain it.
Fighter : Nalia's Keep
Ranger : Ranger's Cabin at Umar Hills
Paladin : Be a part of The Order of the Radiant Heart in the Temple District
Cleric : Depending on what type of cleric, you will get to manage a temple in the Temple District
Druid : Druid Grove
Thief : Mae'Var's Guildhall in the Docks District
Bard : Five Flagoons in the Bridge District
Mage : Planar Sphere
Sorcerer : Planar Sphere
Barbarian : Nalia's Keep
Monk : Nalia's Keep
Magic Resistance (MR)
Magic Resistance is a way for a character to avoid the effects of magic. It is measured from 0-100, although you can have MR higher than 100. Monks and Wizard Slayers get MR as the progress in levels. Also certain monsters like Wolfweres and Dragons have high magic resistance. The wizard spell Lower Resistance lowers MR temporarily.