The game is based on ADnD 2nd edition, which means that the rule system in this game is extremely complex (as compared to games like say, Warcraft 3 or Diablo).
Firstly I'm assuming you're just confused about resistances only. If you don't know anything about hit dice and saving throws and Thac0 and other whatnot available in the character screen, I suggest you refer to an ADnD guide (
http://www.gamefaqs.com has one, go to the Baldur's Gate 1 page (not 2) and look under FAQs and Guides, the link should be there).
Anyway, just to answer your questions: There are 3 different damage types. 1) physical 2)Elemental 3)Magic.
Physical damage has piercing, crushing and slashing, which correspond to melee weapons, and missile for ranged weapons. The girdle of piercing improves armor class against piercing and missile weapons. The orc armor gives missile damage resistance of 10%, which means that for all missile damage incurred 10% will be resisted (i.e. if the missile normally does 10 damage then the target gets hit for 9 hitpoints). Missile adjustment for DEX refers to the adjustment to your Missile AC as well as your ranged weapon Thac0. Finally the girdle of bluntness gives better AC against blunt weapons, which do crushing damage: but not against slings and bullets, which are ranged weapons that do crushing damage. On the other hand crush resistance (like from the Belt of Inertial Barrier) resists ALL crushing damage.
Elemental damage is what it is: damage from fire, cold, electricity,poison, acid, etc. There's no AC versus elemental damage, so a flame arrow will always do flame damage, unless the damage is resisted by resistance (i.e. fire resistance).
Lastly there's magic damage. 3 types: pure magic damage (what you get from spells like Magic missile), magic fire (from spells like fireball) and magic cold. There are 2 types of magic resistances for magic: firstly is magic damage resistance, which negates (depending on %) magic damage taken. The point of differentiating between Mgcfire / Mgccold and Magic damage is because cold and fire damage have special properties (increased damage to opposite element, for example). However, where Magic resistance can negate magic fire or cold, it doesn't do anything towards normal fire or cold damage.
Then, there's magic resistance, which resists the spell itself. I'm not too sure how magic resistance really works; it's almost an Armor Class for spells, but even though magic resistance is in percentage the negation effect isn't down to %. In fact the higher MR the more powerful spells will be negated. Therefore a character with 60% MR won't take damage from magic missiles, but will take damage from high level spells like Comet or Horrid Wilting (then again, maybe not). MR resists enemy spells, including spells like confusion and hold person, but not friendly spells. However some spells ignore MR, like Breach.
For the Iron Golem it's resistant to slashing, piercing, crushing and missile damage by 20%. Perhaps the one you were thinking of was the Clay Golem, which is immune to slashing, piercing and missile damage (100% resistance), but not to crushing (even though it has some resistance).
The game rules can be exceedingly frustrating at the start. But its also the main reason why people enjoy DnD computer rpgs, because there's alot to think about, about which equipment is best (or necessary) and which tactics to use before battle. If you enjoy such things, then happy gaming.
