It is based on the d20 System, with some additions and alterations:
d20 Modern presents a starting character with the choice of six classes: Strong Hero, Fast Hero, Tough Hero, Smart Hero, Dedicated Hero, and Charismatic Hero. These basic classes correspond to the six ability scores used in the d20 System. The basic classes are broad, flexible, and generic, rather than the much more narrowly focused character classes from the Dungeons and Dragons rules.
A d20 Modern character can later, after meeting certain requirements, take levels in advanced classes. The advanced classes are much more specifically focused; examples include Soldier, Field Medic, and Techie.
There are prestige classes as well; these have stricter requirements, which are most likely arrived at through at least one advanced class, and are even more tightly focused in their roles, but these are not found in the core rules.
Also, d20 Modern includes the moreau character race(s), a group of anthropomorphic creatures with individual benefits and drawbacks. This is later expanded on in the d20 campaign setting, GeneTech.
Each character earns a set number of points each experience level, known as 'Action Points'. These points can be spent in game to increase the effect of a single die roll, or to make use of certain abilities earned by the hero character through their experience level advancement.
Upon gaining experience levels, characters earn points which are used to purchase ranks in various skills. These skills quantify in game logic terms the character's competence in some non-combat action, such as swimming, negotiating, stunt driving, or using computers.
Feats are special abilities a character gains. Feats are less readily described because of the sheer variety of abilities they can grant the character. Unlike skills, feats do not have "skill points", but are rather a single "upgrade" you take that grants a bonus of some sort. A feat could allow a character to perform a special combat maneuver, enhance the use of one or more skills, or have some other more exotic effect.
A character can purchase or otherwise obtain any of dozens of items listed in the book, as well as any item that the game master sees fit to allow, using a mechanic which is based on the price of the item. d20 Modern uses a very abstract system for tracking wealth, intended to model modern finances more simply than tracking available funds, credit cards, loan debt, exchange rates, investments, and the myriad other sources of monetary value in a modern society.
Excerpt from an article in Wikipedia.