[Riftmaker]

Okay...detailed description.

When you attack a monster, the game rolls a 20 sided die. If the number rolled, plus any attack modifiers (attack bonuses, target's ac, etc.) is greater than or equal to your ThAC0, you hit the target.

One thing to remember is that if the game rolls a 1, you will always miss, no matter what. Also, if you roll a 20, you wil always hit, no matter what.

Of course, you only do damage if your weapon can hurt the target. Some creatures can only be hit by certain types of weapons, and others can only be hit by weapons that are enchanted to a certain degree.