In (X)
Emacs, this is the area between
mark and
point. Very many
commands operate on the region.
A nice way to check the region is to exchange-point-and-mark (bound to "C-x C-x"). This takes you to the other end of the region, for obvious reasons. To go back, you just exchange-point-and-mark again!
If you use zmacs-regions in XEmacs, you the region painted in some face. And all the commands that work on regions won't work if the region isn't "active". This is meant to help beginning users, but I find it distracting and intensely annoying. Turn that damned thing OFF!!!!1!