I have understood that if you have spotify premium, you can download music there for free? If so, can i dowload any album as mp3 files to a flash drive, and transfer them to for example itunes, and still have the files when my premium is expiered? I couldnt find an answer on google for this.

You can save play lists to work offline, though you can only access and play them through spotify on your phone or laptop.
You can't download them to mp3s or anything like that, just make a playlist offline. It gets synced to your computer into some massive file that you can't acess.

If you have a smarthpone or something you can make a playlist offline for that
No, that would be piracy.

The way it works is that you can make a playlist (I just make playlists that are the actual albums), and save them for offline use. To play the music saved back, you either need to be using the client on PC, and there are iOs and Android Spotify apps which are used. When you go offline, your files on your portable device or PC are valid for 30 days before you need to go back online to update the license.

So it's in between borrowing and owning. There's also a 3333 song limit that you can make offline, and on up to 3 devices.

If you don't plan on streaming using your portable devices, you could get away with paying every second month; go offline on the last day of premium, cancel and you have the music for 30 days until the license expires. Then you get premium every 2 months. I don't stream much on mobile, so could do this, but I'll keep the cash flow going so one day decent royalties are paid.

So yeah, basically it's not a music store, but a way to discover music/conveniently listen to music before you go out and buy the CD. This basically replaces piracy for the "I want to try before I buy" crowd.

Also, fun fact if you're a nerd... Spotify uses Ogg Vorbis files instead of Mp3s and they're 320 kbps, so they will sound better than most of the files available on itunes or Amazon.