Back when discussing the Archwing update with the studio's creative director, Steve Sinclair explained the Relays in relation to players coming to Warframe from more of an MMO background:
We have a lot of players who come from that world. They’re confusing and terrifying to us but we want to make them feel at home! One of the things we’re working on and didn’t talk about is a place – we call them Relays – where people can walk around and talk and meet up to go on missions together. It’s a socialisation place but rather than text chat they’re there with their warframe and kubrow and all this cool stuff. We’re putting effort into growing that part of Warframe and, as you say, it has an MMO feel.
This week those Relays have gone live and can be found in Larunda on Mercury, Kronia on Saturn and Orcus on Pluto. It also sounds like the Relays have their own version of Destiny's Xûr – an elusive trader called Baro Ki'Teer who sells rare artefacts for only a few days at a time. Syndicates sound like factions with whom you can gain reputation (referred to as Standing) by earning medallions and completing missions. There will also be a market as well as places for tinkering with your loadout.
I also need to reinstall Warframe because Sinclair promised me there would be weird-looking space trees and I'm all about that space foliage at the moment.