NickServ - nickname registration "locks in" the use of a nickname to one user. Messaging a bot with a password, prevents other users from using your nickname when you are online. NickServ registration is required for registering channels and sending/receiving memos.

ChanServ - channel registration service. Channels are owned by the network, but can be founded and registered by users. Founding a channel gives a user the right to add and remove ops and akicks, lock channel modes, etc.

MemoServ - A memo service that allows one user to send a message to another, or even to all ops in a channel, even when they are offline. Restricted to users with registered nicknames.