jamCMS is not a CMS itself but a layer on top of other CMS's. In this regard it’s similiar to TinaCMS. However, the main goal of this library is to melt frontend and backend back together in the same way you and your clients are used to it from non-headless websites. But jamCMS is more than that. It's a toolkit for building headless websites more efficiently. The times where the developer had to jump back and forth between frontend and backend code (JS vs PHP) are over. Now you can define post types, templates and fields with code, all in once place, and jamCMS takes care of the rest.
Technically, jamCMS can connect to any data source as long as there is an API which returns the data in the correct format and be used with any React based frontend framework. For the moment though, it is optimized and tested for the use of WordPress and GatsbyJS.
Also, jamCMS is fully integrated with the new gatsby-source-wordpress plugin.