Documentation at http://www.purl.org/tcl/home/man/tcl8.4/TkCmd/button.htm .
A button is a widget which is typically used by a GUI programmer to receive a simple command from the user - a "press here to submit" type idea is typically represented by a button.
Newcomers to Tk often experience several problems whose symptom is that "the button doesn't work right". For details on these sorts of topics, see the FMM [L1 ].
Tk syntax help - Arts and crafts of Tcl-Tk programming Category Command