The nine are the humans that survived the collapse. They are the first-born awoken. They are the only humananoid beings that remember. And since they remember, they keep on building the golden age technologies, or exotics. They send xur (i have no idea who or what he is) to exchange strange coins for exotics. The strange coin is what helps the nine remember what the golden age was. I personnaly believe it was the currency used by humans pre-collapse. I pieced this from grimoire, xur's dialougue, and specualtion.

Same They're ascendant beings that the consensus and the awoken in the reef know about and possibly have dealings with. They seem to be one of the only things that could help us, like they're this all...powerful...All knowing...pyramid...wait ..9...3....9 is divisible by 3.... Oh my god! IT'S LUNCH TIME.[spoiler] I'm sorry, I'm extremely bored. please disregard everything after all knowing. The nine are as I said ascendant beings born from the collapse like the awoken were, they have understanding of the Darkness and therefore in some way power over it. I believe they think neutrality is the best course so they only intervene slightly.[/spoiler]