Software packages for Alpine Linux are digitally signed tar.gz archives containing the programs, configuration files, and dependency metadata. They have the extension .apk, and are often called "a-packs"
The packages are stored in one or more repositories. A repository is simply a directory with a collection of *.apk files. The directory must include a special index file, named APKINDEX.tar.gz to be considered a repository.
More details about Alpine Linux package management are available in the Alpine Linux wiki