The basics of the rules are very simple. Skills use a percentile roll, and combat uses a d20. A character has so many "actions" in a round to be used for either offense or defense. High rolls win d20, low rolls win percentile (holdover from quite a few elder generation games).
The mess part comes from the fact that every character there can have a plethora of different abilities, no two of which are in common between classes (and, yes, it uses a character class/XP/level system). The classes themselves, called Occupational Character Classes (or OCC's), are not balanced against one another. At all. Thus, you can play a normal human scavenger and walk beside a full conversion cyborg, a ley line mage, a mutant from another dimension, and a dragon.
If you can buy into the power discrepancy, and the GM can produce adventures that utilize every PC's skillset, then it is a hella fun time.