Streaming

Streaming media is media that is consumed (read, heard, viewed) while it is being delivered.

http://en.wikipedia.org/wiki/Streaming_media

I.e. you listen the music while your download it. You're maybe not saving the data to Disk but it is still plain downloading.

Anyway, "play the track ... before you download" is an paradox. How can you listen something that you haven't downloaded yet?!

http://en.wikipedia.org/wiki/Time_paradox
http://en.wikipedia.org/wiki/Grandfather_paradox