The Cortex System is based on the Sovereign Stone System. The system described here is the one detailed in the Serenity core rule book. Margaret Weis Productions, Ltd refined and expanded the system for their Battlestar Galactica role-playing game.
The system uses dice with 2, 4, 6, 8, 10, and 12 sides, described with the standard role-playing game notation of d2, d4, d6, d8, and so on. Each of a character's attributes and skills is assigned one of these die types, with larger dice representing greater ability. So, for example, a character might have Strength d6, Intelligence d8, Athletics d4, Guns d10, and so on. When a character attempts an action, such as piloting a spacecraft, shooting a gun, or punching someone, the player rolls the die for the character's applicable attribute and the die for his/her appropriate skill, adds the results together, and compares the total against a difficulty number. Tasks of greater or lesser difficulty are represented either by increasing or decreasing the difficulty number or increasing or decreasing the dice the player rolls by "steps," with each step raising or lowering the die type by one. So a d10 with a two step penalty becomes a d6, a d12 with a one step penalty becomes a d10, and so on.
The game uses a system of Assets and Complications to round out characters and give them various bonuses and penalties. This is similar to the special abilities and character flaws used in a variety of other role-playing games, such as Merits and Flaws in White Wolf's original World of Darkness, Advantages and Disadvantages in Steve Jackson Games' GURPS, Qualities and Drawbacks in Eden Unisystem, and Edges and Hindrances in Pinnacle Entertainment's Deadlands and Savage Worlds games (to which the Serenity system bears a strong resemblance).
The Cortex System also includes Plot Points, which increase characters' survivability and give players greater control over events in the game. Players can spend Plot Points to gain extra dice when making a die roll, reduce the damage from an attack, or even make changes to the storyline. Some Assets also require the expenditure of Plot Points. At the end of a game session, excess Plot Points are converted to Advancement Points, which a player spends to improve his or her character's abilities.
The first two games to use the system were:
Serenity the Role-Playing Game
Battlestar Galactica the Role-Playing Game
Source: Wikipedia, "Cortex_System", available under the CC-BY-SA License.