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, Emacs indicates the extent of the region 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).
This default behavior of the mark is known as Transient Mark mode. Disabling Transient Mark mode switches Emacs to an alternative behavior, in which the mark is always active, which was the default prior to Emacs 23. See Persistent 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 highlight-nonselected-windows is
non-nil, each window highlights its own region.