Overwatch is a fun objective game that is highly team dependent. There are 24 different heroes, divided into four different categories, possible to be.
Currently there is no campaign, but there is quick play, arcade, and competitive game modes. There is currently no explaination why the heroes are fighting each other, there is a reward for fighting which are loot boxes.
Quick play is the basic game mode of the game of a 6 vs 6 set up on a diverse collection of maps.
Arcade is a playlist with an weekly updated list of either: 3 vs 3, capture the flag, mystery heroes, 1 vs 1, no limits, and when there is an event, the special game for it. It also has a server browser that allows you to make or join anyone on the same platforms custom game.
Competitve is a harder version than quick play where instead of doing one round on either attack or defense, you do a round on each side and possibly an overtime. After the game, if you win you can get competitive points that allow you to buy golden weapons.
Loot boxes are rewards that you get whenever you level up or get three of nine wins in arcade. In a loot box you get four items, they can be any of the following: Gold, Skins, Sprays, Voice Lines, Hero Poses, Emotes, and Player Icons.