It's not necessary to explain what online music streaming means at this point. To make everyone understand what Spotify is, all you have to do is say Spotify. It's true that streaming music is the death of MP3 and physical music.
Although the Swedish service is most well-known, Apple Music has also made a name for itself with its large company backing it and a lot of money. We can't ignore Google Play Music and Rdio or Pandora. TIDAL is also available. You can sync your account to other devices by downloading the Android APK and the iPhone version if you sign up for the Windows PC version.
This service is similar to its competitors: million of songs online and the ability to download them to your computer to listen offline. You can also access them with ads, or premium access that doesn't have any adverts. This is a good application, that will meet your musical requirements.
It's an excellent question. You'll find the exact same content in both apps with subtle differences. The Swedes lack Deezer's Flow feature which allows for a more personalized experience and the ability to change the music according to the hour.
Both services are very similar in terms of cost, but we must admit that Spotify's PlayStation Support is superior.
Another question. You can not download the songs to your device in MP3 format. If you are a premium user, you can save music to your computer to be listened to offline . Deezer downloaders or similar programs are not allowed to illegally copy the music, just like Apple Music or Spotify. Did you really expect them to let such an illegal copy of their music ruin their business?