Starting with the iPhone, devices older than the iPhone 5 will not be compatible with iOS 10. This means that iOS 10 will only run on the iPhone 5, iPhone 5s, iPhone 5c, iPhone SE, iPhone 6, iPhone 6 Plus, iPhone 6s, and the iPhone 6s Plus, and while it was not mentioned it goes without saying that this applies to the next-gen iPhone as well, presumably the iPhone 7 and 7 Plus.
As for iPads, iOS 10 will be compatible with just about every iPad out there save for the original iPad. This covers the iPad mini first generation until the fourth generation, the iPad 2, iPad 3, and the iPad 4, followed by the iPad Air, iPad Air 2, and the 9.7-inch and 12.9-inch iPad Pro models. The iPod touch will also receive the iOS 10 update but it is only compatible with the 5th and 6th generation iPod touch.
We guess we should point out the obvious, and that is even though iOS 10 will be compatible with older devices like the iPhone 5 and the iPad 2, it might not necessarily be the best experience as they do sport less RAM, older (and slower) processors, older GPUs, and so on, but we guess at least you have the option of upgrading.