You can try it out for free but that won’t allow you to download stuff, there are occasional adverts & you can only play stuff on shuffle.
I used it for about 4 months or so without paying, but they did an offer around Christmas that was something like 3 months for £3. I’ve been using the paid service ever since, but probably don’t get the most out of it in terms of offline downloads and stuff.
Basically, it’s a massive online music library. There are pre-configured playlists, or you can make your own.
Like I say, the free version only lets you stream music, so you either need an unlimited data allowance or use it only over Wi-Fi.
Once you pay for it, you can download what you want and it is available on your device for however long you pay for it. Once you stop paying for your spotify subscription, you lose access to that downloaded music.