Once you have a region, here are some of the ways you can operate on it:
Some commands have a default behavior when the mark is inactive, but
operate on the region if the mark is active. For example, M-$
ispell-word) normally checks the spelling of the word at
point, but it checks the text in the region if the mark is active
(see Spelling). Normally, such commands use their default
behavior if the region is empty (i.e., if mark and point are at the
same position). If you want them to operate on the empty region,
change the variable use-empty-active-region to t.
As described in Erasing, the DEL
backward-delete-char) and delete
delete-forward-char) commands also act this way. If the mark
is active, they delete the text in the region. (As an exception, if
you supply a numeric argument n, where n is not one, these
commands delete n characters regardless of whether the mark is
active). If you change the variable delete-active-region to
nil, then these commands don’t act differently when the mark is
active. If you change the value to
kill, these commands
kill the region instead of deleting it (see Killing).
Other commands always operate on the region, and have no default
behavior. Such commands usually have the word
region in their
names, like C-w (
upcase-region). If the mark is inactive, they operate on the
inactive region—that is, on the text between point and the
position at which the mark was last set (see Mark Ring). To
disable this behavior, change the variable
nil. Then these commands will
instead signal an error if the mark is inactive.
By default, text insertion occurs normally even if the mark is active—for example, typing a inserts the character ‘a’, then deactivates the mark. Delete Selection mode, a minor mode, modifies this behavior: if you enable that mode, then inserting text while the mark is active causes the text in the region to be deleted first. Also, commands that normally delete just one character, such as C-d or DEL, will delete the entire region instead. To toggle Delete Selection mode on or off, type M-x delete-selection-mode.