Apple noted in its iPad PR that iPadOS will roll out to older iPads on September 30th. As for what devices are supported: all iPad Pros, the iPad Air 2 from 2014 and later, the 5th-generation iPad from 2016 and later and the iPad mini 4 from 2015 and later.
Here are some of the new features. Multitasking features have been significantly improved for one. You can now have multiple instances of supported apps in different “spaces” imagine having a Safari window in split-screen view alongside Notes, and another next to your Mail apps, for example. The home screen has been slightly changed so you can see more apps and pin widgets to it. Naturally, it’ll get all the updates that iOS 13 will provide to iPhones, as well that includes improved performance across the board, a system-wide dark mode and major updates to apps like Maps, Reminders, Photos and Notes.