Function: delete-selection-mode

Toggle Delete Selection mode.
With a prefix argument ARG, enable Delete Selection mode if ARG
is positive, and disable it otherwise. If called from Lisp,
enable the mode if ARG is omitted or nil.

When Delete Selection mode is enabled, Transient Mark mode is also
enabled and typed text replaces the selection if the selection is
active. Otherwise, typed text is just inserted at point regardless of
any selection. (fn &optional ARG)

Variable: delete-selection-mode

Non-nil if Delete-Selection mode is enabled.
See the command `delete-selection-mode' for a description of this minor mode.
Setting this variable directly does not take effect;
either customize it (see the info node `Easy Customization')
or call the function `delete-selection-mode'.