Just as cameras are used in films to display the story to the audience, Cameras in Unity are used to display the game world to the player. You will always have at least one camera in a scene, but you can have more than one. Multiple cameras can give you a two-player splitscreen or create advanced custom effects. You can animate cameras, or control them with physics. Practically anything you can imagine is possible with cameras, and you can use typical or unique cameras to fit your game's style.
The remaining text is from the Camera Component reference page.
Cameras are the devices that capture and display the world to the player. By customizing and manipulating cameras, you can make the presentation of your game truly unique. You can have an unlimited number of cameras in a scene. They can be set to render in any order, at any place on the screen, or only certain parts of the screen.