Substack is a publishing and paid subscription platform. It is most often thought of for newsletters, but it’s more of a combination newsletter + blogging platform.
It is a fully hosted, proprietary, free to start platform. They charge 10% of fees you collect.
As of October 2020, you can pay $50 one-time to add a custom domain.
Ghost is an open source blogging and publishing platform.
More recently, it’s been calling itself a [[Headless CMS]]. You can use Ghost for its admin and editing interface for making blog posts and pages, and then use a different framework such as [[11ty]]1 on the front end to customize and display the content.
Ghost is now also being compared to [[Substack]], it has a Ghost vs. Substack Comparison Page. This is powered by Ghost’s membership / subscription features.
Ghost was crea... has a Ghost vs. Substack page that goes into more detail to consider.
My personal recommendation if you’re going to use Substack, is to use it with a custom domain. Then, all the links going to your content are portable should you switch platforms later.