Java Edition server software

Java Edition server software allows for players to play on multiplayer servers. Official Java Edition server software includes Realms, with unofficial software being plenteous. Some unofficial server software can be found below.

Official Server Software
With every release, Mojang releases official server software. This is always available with every major release and snapshot.

Plugins
Some un-official software allows you to run plugins natively in the game. These plugins and the software used to run them are not run or maintained by Mojang Studios.

Modded
Some un-official software allows you to run modded versions of the game. These mods and the software used to run them are not run or maintained by Mojang Studios.

Plugins + Mods
Some un-official software allows you to run modded versions of the game as well as plugins. These mods, plugins and the software used to run them are not run or maintained by Mojang Studios.

Proxies
Some un-official software allows you to chain multiple servers together behind a proxy. This allows you to switch servers without actually leaving.

Protocol Translators
Some un-official software allows you to connect to Bedrock edition servers from Java Edition or vice versa.