Apple has consolidated its developer program, bundling together access to developer tools and pre-release software builds for iOS, OS X and watchOS all under one umbrella. The unification of the dev programs is very much in favor of developers; previously, the Mac and iOS programs each cost $99 per year for access, but now it’s just $99 annually for both, plus the new watchOS native development tools.
If you’re already a registered developer who belonged to either the iOS or Mac dev programs, Apple has already transitioned your membership to the new model, meaning you have access to all after you accept the new terms and conditions. There’s also an automatic renewal program coming soon, per the developer portal page.
You can see an overview of the new dev program here, or find out what’s included in your membership here. For those who are already members and want to learn what’s changed, take a look at this guide to the transition here, which tells you things like what happens to teams, etc.