Per the ongoing stream, Destiny 2 will have clan support, including in game rosters and custom banners. Destiny players have been unofficially grouping up as communities, but now official clan support means you can do that in the game itself.
Clans will have shared reward systems, so you all contribute and receive rewards together. Kind of like sharing tips.
Guided Games, meanwhile, is a mode where solo players can be paired with clans. Like a big brother system, Clans are planned to give solo players the chance to experience more challenging battles and raids in Destiny 2 where they couldn't in Destiny. It also means if your Clan only has, for example, five players, and the raid needs six, then you can find the final player you need. This is different to trad matchmaking. We think. Probably.
Maybe the real Clan was the friends we made along the way, everyone.