libbrillo is a shared library meant to hold common utility code that we deem useful for platform projects. It supplements the functionality provided by libbase/libchrome since that project, by design, only holds functionality that Chromium (the browser) needs. As a result, this tends to be more OS-centric code.
This project is also used by Update Engine which is maintained in AOSP. However, AOSP doesn't use this codebase directly, it maintains its own libbrillo fork.
To help keep the projects in sync, we have a gsubtree set up on our GoB: https://chromium.googlesource.com/chromiumos/platform2/libbrillo/
This allows AOSP to cherry pick or merge changes directly back into their fork.
