Heroes is a new way to play Action. While it is based off of teamplay code, it's not really a team oriented mod. There are many features from teamplay, like the two teams, but it is every man for himself.
Team 1 consists of one player, the Hero. Players take turns being the Hero. The Hero starts the round with his Hero Weapon, which is decided by the serverop. The Hero doesn't follow the normal Action rules:
Team 2 is the Villains. Villains follow all the normal Action rules. They start the round with whatever weapon the player chose in the menus. They can't kill the Hero, but they get points for damaging him. Everytime a Villain accumulates 100 damage points against the Hero, he receives a frag. Damage tallies are kept through rounds. A Villain receives full credit for a shot on the hero, even if it results in more than 1 frag.
The object of the game is to accumulate the most frags, by inflicting damage on others when they are the hero, and by not allowing damage on you when you are the hero. Inflicting damage on Heroes requires mostly the same skills as Action, and the realization that the Hero will return fire no matter what. It's not possible to merely hit first and kill him. Being the Hero requires different skills. You can elude your attackers by doing things they can't, like jumping off buildings. You can be more liberal with ammo, and you'll need to be, with so many attackers. Be careful, though; you still need to reload. Winning the game requires you to play both sides well.