1D20  (20 Sided die)
    This is rolled when there's a contest between two characters...  Kinda like Paper-Rock-Scissors
Bob attacks Jim.  They both roll a 1D20 (1 20 sided die) and the higher number wins -in a tie, the defender always wins

_D4, _D6, _D8, _D10  (4, 6, 8 ,10 sided dice)
    These are typically rolled when damage from an attack is sustained.
    -Every weapon has a "Possible Damage Range", like 2D4.  So, this weapon can do from 1 to 8 points of damage.
        -Since not every blow inflicted will cause maximum damage, this is a random way to decide how much damage was done.
Bob hits Jim.  He hacks him with his sword that does 1D6 damage.
Bob rolls 1D6 -and gets a 5.  So with that swing, Bob inflicted 5 points of damage.
    -The damage roll is rolled every time Bob hits Jim.

1D100  (100 sided die) -yes, these really do exist!  they are large and the number surfaces are very small...
                                                                                                                 it looks like a golf ball with numbers. 
    This is rolled when the character is playing the odds...  like when performing a skill, or gOD decides there's "a chance" that something might happen.
Bob decides that Jim ain't such a bad guy, and he shouldn't have been picking on him.
    He injured Jim though, so Bob wants to help Jim.
Looking at his Skill List, Bob sees that he has a 30% chance of doing something beneficial for Jim with his 'Medicine' skill.
Bob Rolls his % die (1D100, or 100%)
    Since your skills improve with practice, going up in probability that you'll do it right, you have to roll under your skill number.
        -So as your skill % number goes up, it gets easier to roll under it.
Bob rolls a 63.  Bad news for Jim.  After botching it and hurting Jim, Bob rolls again.  This time he rolls a 10.  Since that's under 30, he does it successfully, and Jim is patched up.
Though, Jim's probably still angry at Bob for stabbing him in the first place, and then botching up the first attempt at aid.
 
 

SO, to sum it all up.
Roll a 1D20 for competitions, Roll a 1D100 for odds and roll everything else depending on what the weapons calls for!