Anyone play Baldur’s Gate II?

I downloaded the demo and, since I’m somewhat new to this style of RPG, I’ve got a couple questions. When I equip armor, the number on a shield icon in the inventory decreases. What does that mean? Also, in my character’s stats, it will say something like "Armor: -2 Piercing" or "+2 Crushing." What does that mean? (I’m guessing it means how well the armor protects you against different types of attacks, but is + or – bad or good?) Oh, and if my character is skilled in two weapon combat, how do I make her equip two weapons?