OSTree is a tool for managing bootable, immutable, versioned filesystem trees. While it takes over some of the roles of tradtional "package managers" like dpkg and rpm, it is not a package system; nor is it a tool for managing full disk images. Instead, it sits between those levels, offering a blend of the advantages (and disadvantages) of both.

For more information, see:

https://live.gnome.org/Projects/OSTree