What Is Apple TV?

Apple TV refers to the hardware device.

Apple TV+ refers to the streaming subscription service.

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A bit of both actually.

• There have been 5 generations of ‘Apple TV’ set-top boxes now. These devices connect to a TV by HDMI, to provide Apple’s version of smart TV (and bypassing any built-in smart functionality of the TV). The later generations allow custom third party apps from an on-device App Store, somewhat similar to iPad. The Apple TV box will provide for AirPlay receiving capability.
• There is an Apple TV app, to provide some core functionality and hub functionality for select streaming services, including iTunes Store purchases. The app will run on devices with iOS 12.3 or later, tvOS 12.3 or later, macOS 10.15 or later, and now even on select third party devices). Functionality differs a little bit depending on device.
• There is Apple TV+, the streaming service featuring Apple Originals.

Also see: Apple TV app vs. Apple TV box vs. Apple TV+ - Apple Support Communities