Apt-Cacher-ng is a software package that keeps a cache, on the disk, of Debian/Ubuntu Packages and Release files. When an apt-get like client issues a request for a file, Apt-Cacher intercepts it and if the file is already cached it serves it to the client immediately, otherwise it fetches the file from the Internet, saves it on the cache, and then serves it to the client. This means that several Debian machines can be upgraded but each package need to be downloaded only once.