Operator

An operator, also known as a moderator, is a player designated to moderate a multiplayer server by the admins and the owner of the server. This is done to help keep control of the server from unruly players and griefers, and even a reward to trusted players. People are made operator by using the  command, or by editing the "ops" text file in the server, then restarting the server. Operators have access to a variety of server commands which aid them in their policing duty. These are activated by typing them into the chat with appropriate arguments.

Operators also are granted the privilege of destroying and placing blocks in the spawn area; although the admin can also decide to let all players destroy and place blocks.

Problems with untrustworthy operators
Operators have access to all commands meaning they can stop the server, use the  command, change game modes, and kick/ban players including the admins and server owners. This can be avoided in modded servers with a permissions plugin. This makes some operators unable to use certain commands.

On SMP servers, untrustworthy operators can also be tempted to go into creative mode. This can result in griefing, an unfair balance, and anger among the players. Once again, this can be combated using a permissions plugin.

Operators are also able to edit command blocks, so be wary that they may be changed by untrustworthy people.