Spotify works on mobile devices with an app, and on computers with a downloadable app for Windows and Mac, and a web app for Chromebook. It's also just been launched in Vizio Smart TVs. The free level lets you choose from their pre-set "shuffle" playlists and is ad-supported, while the premium level ($9.99 a month) allows you to choose any song you like to play, and also lets you download and listen offline. Spotify does not have a free level on the smart TV.
Spotify's differentiation is their "music discovery" and social sharing of music and playlists with your friends who are also Spotify members. (I don't think I have any, lol.) Spotify also has developers who create extensions to build special playlists and such things. This makes it something with lots of surprises when you poke around, but not my first choice.