ChanServ - Channel management, prevents take overs and the like. Use NickServ to register an account. /cs help for help.

NickServ - To register an account and your nicknames thus preventing others from using them. /ns help for help.

MemoServ - If a user is offline and you want to send him/her a message, you can leave one using MemoServ. /ms help for help.

There are other services, like Global, but the ones mentioned above can be used by regular members.
All of the services are modified from x3.