AppStream is a cross-distro effort for building Software-Center applications and enhancing metadata provided by software components. It 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.
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 AppStream distro-repository data is stored in a Xapian database, which then contains information about all available components in the distribution's software repositories.
All parts of AppStream are distro-agnostic (with exception to some metadata which is used to generate the Xapian application cache), and therefore AppStream makes it possible to build distro-agnostic application management tools.
