A KeyServer is a server which processes and contains PGP keys.
KeyServers, such as the ones used by Debian and Ubuntu, are a key component in being able to authenticate packages, and other software. They are a key component in the making of Debian installers (.deb packages), and can also be used for encrypting content. KeyServers allow for users to import other PGP keys from other users and sources so that verification and decryption of information sent by others can succeed.
