ADF, or Application Development Framework, is a set of Angular components to build various types of applications backed by Alfresco products. It's not the application itself so it is not replacing anything.
The repo tier doesn't have a UI. ADF-based clients like ACA and ADW are being actively developed now. You can run ACA/ADW side-by-side with Share (the UI most folks use currently), but my advice is to not spend any significant effort creating major customizations to Share from this point forward unless they have a 3 year payback or less.
