Many Emacs commands operate on an arbitrary contiguous
part of the current buffer. You can select some part of the buffer and
edit only that part of the buffer. This selected buffer is called a
region. You can select text in two ways:
You use special keys to select text by defining a region between the
cursor and the mark (which you set).
If you are running SXEmacs under X, you can also select text
with the mouse.
Selecting Text: Select a region of text by setting the Mark