"Message(s)" used alone will display all of the stored messages that have been sent to you.
"Message(s) user" will display all stored messages from you to that user and from that user to you.
"Message(s) user text" will send "text" to the user.
All messages will be stored until they are cleared by the recipient. In contrast, "tell"s are not stored.
See also: clearmessages, tell