Hey Ken. So they are used to talk about what you used to do, and what you were doing. The main difference being that the 'imperfect' is a little more vague and may imply that the duration of the action is spread over a long period of time, whereas, the 'imp progressive' is specific and refers to an action that happened at an specific point in time (much like it does in the progressive indicative).
The progressive is often used to emphasize actions that were happening while something else happened. It is often interrumpted by other actions.
ex: Yo trabajaba en el cine (I used to work at the theater) - simple fact, we dont know for how long, it might have been for a couple of weeks, years, etc.
Yo estaba trabajando en el cine cuando robaron el banco de al lado (I was working at the theater when they robbed the bank next door.) - is a progressive action, also specific, something I was doing when something else happened.
Yo me acordaba de ti siempre que escuchaba esa canción (I would think of you every time i would listen to that song)- every time the song came on.
Yo me estaba acordando de ti al escuchar esa canción (I was thinking of you while listening to that song) - at that very moment in time.
hope that helps. :)