Battle.net is Blizzard's online portal for their games, including the MMORPG World of Warcraft, and Overwatch, Blizzard's most recent success.
Destiny wasn't available on PC, and it makes sense that Destiny 2 comes to it. That it arrives via Battle.net is, uh, something of a surprise. Though Blizzard is also part of the Activision family, this is the first time a non-Blizzard title has been on Battle.net.
Update: Blizzard has updated its website with some further details. Destiny 2's servers and customer service will all be maintained by Bungie, but it'll be integrated into many of Blizz's existing social features like chat, and adding friends or forming parties. You can also buy Destiny 2 with Blizzard Balance.
Blizzard also explains the decision to bring an outside game to its own platform:
'Creating a new network client for Destiny 2, which is bringing the franchise to PC for the first time, would needlessly extend the development period for the game. We want to get our hands on Destiny 2 as soon as possible like everybody else, so we offered to share our PC platform with our sister companies for this release.'
Despite this, Blizzard confirms it has no immediate or long term plans to add more third-party games, 'Aside from potentially evaluating needs or opportunities for future Activision games'.