Explain Like I'm 5 (ELI5)

Some people describe it as a network, some people describe it as a forum, yet somehow we have providers, and at the same time Usenet is seen as anonymous despite having a proxy in the form of a provider...for someone grown way after the Usenet prime, this all doesn't make sense.

What is Usenet, actually? Is it a separate network? Is it layered on top of the Internet? If it's the latter, why do I need some Usenet provider and why does it need to retain information (and why doing it for as long as possible is desirable?). Please help me connect the dots here.