NickServ allows users to 'register' a nickname, and stop others from using that nick. NickServ allows the owner of a nickname to disconnect a user from the network that is using their nickname.
If a registered nick is not used by the owner for 30 days, NickServ will drop the nickname, allowing it to be reregistered.
For more information on a command, type:
/msg NickServ help <command>
For a verbose listing of all commands, type:
/msg NickServ help commands