Many Emacs commands operate on an arbitrary contiguous part of the current buffer. To specify the text for such a command to operate on, you set the mark at one end of it, and move point to the other end. The text between point and the mark is called the region. The region always extends between point and the mark, no matter which one comes earlier in the text; each time you move point, the region changes.
Setting the mark at a position in the text also activates it.
When the mark is active, we say also that the region is active; Emacs
indicates its extent by highlighting the text within it, using the
region face (see Face Customization).
After certain non-motion commands, including any command that changes the text in the buffer, Emacs automatically deactivates the mark; this turns off the highlighting. You can also explicitly deactivate the mark at any time, by typing C-g (see Quitting).
The above default behavior is known as Transient Mark mode. Disabling Transient Mark mode switches Emacs to an alternative behavior, in which the region is usually not highlighted. See Disabled Transient Mark.
Setting the mark in one buffer has no effect on the marks in other
buffers. When you return to a buffer with an active mark, the mark is
at the same place as before. When multiple windows show the same
buffer, they can have different values of point, and thus different
regions, but they all share one common mark position. See Windows.
Ordinarily, only the selected window highlights its region; however,
if the variable
nil, each window highlights its own region.