Game Mechanics

We are talking pretty much the standard D20 system here. When a character attempts an action with any significant chance of failure (Such as firing a gun or hacking a door not something like brushing ones teeth) they should attempt the following test. If the result equals or beats the target number the test is successful. If the result is lower than the target number the test has failed.
 * 1) Roll a D20
 * 2) Add/subtract appropiate modifiers
 * 3) Compare to target number

Simple right?

Well there's plent more ofr complexity sake but no one cares so we wont worry about that.