Ban
A ban is when a player has been banned or banished from a server by its admins. A banned player will no longer be able to return unless they either are unbanned by an admin or they access the server via a new IP.
In JKA, bans are handled by IP, this means that when a player is banned, their IP is added to a list of banned IPs, and anyone attempting to connect from that address will be unable to connect. For this reason, bans are not effective against players who have a Dynamic IP Address.
Range Bans
Admins can manually block entire ranges of IPs, usually as an attempt to ban players with dynamic IPs, or to block entire countries. Depending on how the internet service provider responsible for the IPs distributes them, this can result in other players being accidentally banned, usually, players from the same geographical region as the targeted player. It is usually only used as a last resort.