http://purl.org/tcl/home/man/tcl8.4/TkCmd/message.htm
Roughly spoken, a multi-line label which can reformat itself to shrink-wrap the contained text. See a little fortune teller for a usage example.
How does message differ from tk_messageBox? In vernacular English, tk_messageBox is aimed at a situation where a user needs to see a message, and respond to it before proceding, while message is more of a "legend" that can appropriately stay on-screen or otherwise "within reach" over an extended interval.
RS: tk_messageBox is a toplevel dialog that contains a message and one or more buttons (OK at least).