As it stands there are 25 different heroes to choose to play in Overwatch. Blizzard are continually adding new heroes into the game, each hero is initially released into the PRT (Public Test Realm) server before being added into the game for competitive play.
The game currently has four different types of hero; offensive, defensive, tank and support. A balanced team should have a mix of all of these different categories and it is important to note that it is not possible to utilize two of the same hero in competitive play. Each of the heroes has a variety of different abilities that are either available continually, or on a cool-down. Additionally, each hero has an ultimate ability that has the potential to turn the tide of any game. Ultimate abilities are charged over time, based on the performance of the hero throughout the game. Some ultimate abilities are defensive, while others are offensive.