iTunes is a media player, media library, and online radio broadcaster created by Apple Inc. It allows users to play, download, and organize digital media files like music and videos on personal computers running macOS and Windows. Additionally, iTunes can be used to buy music, movies, TV shows, and audiobooks from the iTunes Store.

Yes, iTunes is available for both Mac and Windows. Apple offers a version for Windows so users can enjoy the same features as Mac users. It can be downloaded from Apple's official website or the Microsoft Store for Windows 10.

While you can listen to media files stored locally on your computer without an Apple ID, you need an Apple ID to access and buy content from the iTunes Store. The Apple ID is also required for downloading media and accessing iCloud and other Apple services.

To back up your iPhone with iTunes, connect your device to your computer and open iTunes. When your device appears, select its icon. Under the 'Summary' tab, click the 'Back Up Now' button to start backing up. Make sure your computer has enough storage space for the backup.

The key difference is their primary purpose. iTunes is a media library and player where you can purchase and own media permanently. Apple Music is a subscription streaming service offering access to a large music library without ownership, allowing streaming and offline listening as long as the subscription is active.