Lighting, Light Fixtures, Ceiling and Exhaust Fans - New work junction box vs Old work junction box?

What is the difference between an "old work" junction box and a "new work" junction box? Is a "new work" junction box only for use in a house that is of new construction? Is there some reason a "new work" box can't be used to add a junction box in an old home? I saw junction boxes at my local home store labeled "new work" and "old work" and I want to know if I must use an "old work" one in my old (c. 1929) house.

I need to install a round junction box in my bathroom in order to install a flush mount ceiling light fixture. The old fixture was a recessed florescent strip light that has been removed.

New work box is designed to fastened to the studs when no wall board is up.
Old work box is designed to be installed into existing wallboard. It has mounting ears to make this happen without damaging the wallboard. There is no reason not use new work boxes in old homes. It all depends on the access you have for the method of mounting the box.

Good news. I think a "new work" junction box is just the thing I need to hang a ceiling flushmount light. The rafters in my attic are 2 x 10's, so I have a lot of room to get access to hang the junction box. Assuming that the "new work" juntion boxes extend a full 16 inches wide, that should do the trick. That way I don't have to cut wood supports to size. Thanks for the info.