AppStream metainfo.xml files are like extended .desktop files used to populate app stores with information about GUI applications. Debian and Ubunto and other major Linux distributions are starting to push for their inclusions. This is a fairly minimal example that just covers the required entries. The text for the description is a slightly modified version of text taken from Electrum's FAQ.
Metainfo files can contain links to screenshots. I have not included any here, but that might be something someone would like to add in the future.
sorenstoutner