What are /r/Prog's favorite Products & Services?

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I don't know about the definition, but, if you want to hear examples (and sub-genres) of progressive music try Every Noise. Click on List and scroll down to line number 252 Progressive Rock. It gives you a Spotify playlist. There you go!

Warning; This website is an Extremely deep and fun rabbit hole, you will find and hear genres that you never knew existed (I mean WTF is Glitchcore?).

Enjoy!

If what you're talking about is scoring, I think your best bet is MuseScore. Back in the day I used to use Flat.io which is fine, I guess, but they limit the number of scores you can create (that shouldn't bother you until you reach 15 scores but I say just save yourself the trouble and learn MuseScore).
Anyway, if I'm not mistaken, MuseScore lets you listen to synthesized versions of all the instruments. It sounds pretty bad but it is useful for developing harmony and hearing how stuff sounds at a basic level.
Also if you're into paying you might be interested in Sibelius. I have no experience with it but people say it's good I think.