They're available right now, and here are a few of the highlights:
For iOS 12.1, the big changes are as follows:
The big additions for watchOS are:
Note, however that the much-anticipated ECG app is not yet available to users in the United States. Unfortunately, there's no firm timetable for when it will be, but the company promises that it will be soon.
Information is a bit spotty where the details of the tvOS update are concerned. This indicates that it's a more modest offering, containing bug fixes for existing issues rather than introducing any new features.
HomePod also got an update, although the notes for that one were sparse too, indicating little more than "general improvements for stability and quality."
In any case, if you own one or more Apple devices and you're not set up for auto updates, it's well worth paying a visit to Apple's website and checking out the latest options. Odds are that you'll want them.