Can someone explain a couple things to me?

You may also see the term "secondary double glazing."     That's where extra windows have been installed inside the existing single-paned windows.  The extra window is typically fitted into the existing recess of the window opening and is spaced several inches in front of the original window, compared to the half-inch or so which is normal between panes for a fully sealed DG unit.  The secondary unit is often provided with sliding panes, so you can just slide one back and open the original window.

Secondary DG was a popular option some years ago to provide some of the benefits of DG at cheaper cost. 

By the way, don't get too excited by that pic of the AGA. Unless you're quite well off, you won't see one in any property you look at I bet! Get your mind ready for 'efficiency apartment' living. We live in a so-called 'luxury flat' and our kitchen is so small I had to take the door off the hinges and stick it in the hall closet because when you opened it, it hit the cooker (ie, oven). LOL

By the way, don't get too excited by that pic of the AGA.

I wouldn't get excited anyway-our friends have one and you can't adjust the heat in the oven, and they dry foods out terribly.  I always wanted one untl I tried theirs!