AppStream is a cross-distro effort for enhancing the metadata available about software components in the Linux and free-software ecosystem. One of the project's goals is to make building software-center applications possible, and make interaction with the package sources of a distribution smarter. AppStream provides specifications for meta-information which is shipped by upstream projects and can be consumed by other software. The meta-information includes data which is interesting to display in software centers and is mainly useful for end-users, as well as descriptions about the public interfaces a software component provides, which is mainly useful for developers, 3rd-party software installers and for automatically installing missing components on a distribution, for example missing firmware or mimetype-handlers.
Distributors provide metadata as well, which describes all components available in a software repository. That data is composed of the upstream-metainfo and some other sources. AppStream also provides specifications for things like a screenshot-service, application ratings & reviews etc.
All parts of AppStream are distribution-agnostic, and therefore it is easily possible to build software management tools with it that work on any distribution implementing the AppStream specification.
