OSTree is best summarized in a single sentence as "git for operating system binaries". At its core architecture is a userspace content-addressed filesystem, and layered on top of that is an administrative layer that is designed to atomically parallel install multiple bootable Unix-like operating systems.

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, OSTree sits between those levels, offering a blend of the advantages (and disadvantages) of both.