On a Bungie blog post, Senior Designer Tim Williams explains that factions of the Tower will rally to collect resources for its cause. This involves both gathering their own supplies... and destroying the enemies'. Do a good job and you'll be suitably rewarded. According to Williams: "As an incentive, each faction has set aside a powerful weapon to entice players to choose their side. The faction whose Guardians collect the most faction packages will be declared the winner."
The eventual faction that wins will offer the weapon to everyone, but if you picked the right side to support you'll land a huge discount. Oh, and you must already be level 20 character, with access to the Tower. Once you've picked your faction, you can assist through public events, Lost Sectors, strikes, raids, or simply winning in the Crucible. Wins mean tokens, and tokens can be turned into loot.
The official blog post details what's on offer, but rest assured, there's plenty of items on offer, regardless of your affiliation. You'll be able to save up faction tokens until 2AM Pacific on October 3, with the eventual winner declared eight hours later, at 10AM Pacific time. While it's a temporary event, it's a nice touch to see the return of the factions, with a more interesting proposition that the rep system of the original, which depended solely on individual players' actions.