What is the difference between Past Perfect Progressive and Past Progressive?

OK. I'll give it a go.

Past progressive represents an activity taking place prior to the present, i.e., prior to the moment of speaking.

Past Perfect Progressive represents an activity taking place prior to the past, i.e., prior to some moment in the past (usually mentioned in the same conversation or even in the same utterance).

In the following examples, past progressive appears in italics; past perfect progressive appears underlined.
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-- What were you doing when you heard the loud noise?
-- I was walking toward the kitchen.
-- Why were you walking toward the kitchen?
-- I had been watching television for some time and had gotten hungry. I remembered that I had left part of a sandwich in the fridge and had been thinking about how good it would be to finish it off.
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-- Someone stole my wallet while I was having lunch at the sidewalk cafe on Elm Street.
--Who do you suppose stole your wallet?
-- I'm not sure. A very strange looking woman was pacing back and forth along the street. When I noticed my wallet was missing I asked the owner what he knew about her. He merely replied that she had been pacing like that for several hours but that she was harmless. I decided she was not the culprit.