How select a rectangle in emacs

In Emacs 24.4 and greater, C-x SPC is bound to rectangle-mark-mode which selects a rectangle visually.

http://emacsredux.com/blog/2014/01/01/a-peek-at-emacs-24-dot-4-rectangular-selection/ describes this in greater detail.

One more fun one to add to the list.

If you're using evil-mode (ie, the Vim emulation layer), you can use evil-visual-block (default binding of C-v) to select a rectangle visually.

In Emacs, whether a command affects a continuous piece of text or a rectangle depends on the command, not on what the selection looks like.

I'm not aware of any way to make the selection appear as a rectangle on the screen (but see this answer), but if you use any of the rectangle commands listed in the page you linked to, such as C-x r k or C-x r t, you'll see that they operate on the rectangle defined by the beginning and the end of the selection, despite the fact that the selection looks like a regular region.