MemoServ is an IRC service that allows users to send messages between registered users. MemoServ is typically used for short communication, or for leaving a message for someone who is currently offline or away.
When a memo is sent, the receiving user will get a notice of a new message both in their current IRC session, and when they sign into NickServ again if they ignored the immediate notice.
MemoServ is also one of the lesser used services on the network, but once again, does serve a purpose.
All help for MemoServ can be accessed by typing /msg MemoServ help. You can always join #help to ask a member of network staff for help.