Overview
Mutants & Masterminds is a superhero role-playing game written by Steve Kenson and published by Green Ronin Publishing. The current 3rd Edition, which also serves as the engine for the DC Adventures game, was released in 2010. It is highly flexible, point buy system that allows players to build any superpower they can come up with, from classic caped crusaders and vigilantes to cosmic entities and mystical sorcerers.
The system it directly evolved from is
d20 System (2000)
Description
Mutants & Masterminds' game mechanics use a highly modified version of the d20 System. All characters in this game, from heroes and villains to the average person on the street, are defined by eight abilities, basic traits each character has to a greater or lesser extent. Abilities tell you how strong, smart, and aware a character is, among other things. The abilities are: Strength, Stamina, Agility, Dexterity, Fighting, Intellect, Awareness, and Presence
System Overview & Key Features
The Single d20 Engine
The system is a true d20 engine. The twenty-sided die will be the only one you will need for the rest of the game. Every roll happens with the d20 as the baseline and then various bonuses and modifiers are added or subtracted.
Point Buy Character Creation
There are no random rolls for character creation. Players are given a budget of Power Points based on the campaign's Power Level. Every ability, from super strength to immortality, has a specific cost per rank, enabling high amounts of character customization and forging your unique superhero (or -villain).
Effects Based Power Design
Instead of a list of hundreds of pre defined spells or powers, the game uses Effects. If you want a fire blast, a freezing ray, or a telekinetic shove, you simply buy the "Damage" effect and apply descriptors like "Fire" or "Cold". This allows for endless creativity in how powers are themed.
The Toughness Save vs Hit Points
The game completely eliminates Hit Points. When a character is hit, they must make a Toughness resistance check. Failing the check results in cumulative penalties. A minor failure causes a "Bruise" penalty to future saves, while a massive failure can result in being dazed, staggered, or instantly knocked unconscious.
Power Level Caps
To keep the game balanced, the Game Master sets a Power Level (PL) for the campaign. This acts as a ceiling that limits the maximum total of a character's attack bonus, damage, defenses, and skills. This prevents one player from being mathematically untouchable while others struggle to contribute.
Victory Points and Heroics
Players can earn Victory Points (formerly Hero Points) for roleplaying their character's complications, such as a secret identity or a specific weakness like kryptonite. These points can be spent to reroll dice, recover from injuries, or use "Power Extractions" to perform stunts they haven't officially bought yet.
Additional links
greenronin.com 
https://greenronin.com - Official Green Ronin Publishing website
d20herosrd.com https://d20herosrd.com - Official Mutants and Masterminds System Reference Document