RMSS uses a percentile dice system and uses both classes and levels to describe character growth. Character generation can take quite a long time if a player doesn't know the rules well, and even experienced RMSS players can take over an hour generating a new character. Often derisively called 'Chartmaster' or 'Rulemonster' for it's dependence on a truly bewildering number of different charts and rules for determining quantity and quality of success. Skills are very detailed, sometimes taking multiple skills to perform a single action.
Rolemaster is also renowned for (others might say 'infamous for') its use of 'Critical Hits'. When in combat a person will find on the weapon charts primary damage as well as secondary damage in the form of a Critical Hit. By rolling on (yet another) chart a player can determine this secondary damage, which can be anything from your opponent laughing at you if you roll poorly at you to instant death of the target if you roll well. The fact that opponents also roll on these same charts makes RMSS a very deadly game since at any given time an opponent can get lucky and then a completely healthy character is instantly dead . Also, many rolls are open-ended, meaning that when a number between (and including) 96-100 is rolled, another roll is performed and added to the previous result. If another number between 96 and 100 is rolled, this happens again until the player or game master throws smaller number than that. This can make even an usually harmless player or non-player character perform occasionally surprisingly good actions. On the other hand, if a roll between 1-4 results, then another roll is done and substracted from the initial result. This causes even usually superbly skilled characters to have occasional severe failures in their actions.
The rules for RMSS don't come so much in a single base book as they come spread out among many source books. It's easy to tell which books are called this as the last word in the title is always ...'Law', thus the book on magic is called 'Spell Law' and the book on weapons is called 'Arms Law'. This spreading out of required material with the dependence on charts can make the game burdensome if a trip to the nearest copy machine isn't made. This also leads to an additional derisive name for the system: 'Arm Slaw'.